Our toddler is a climber. I mean, a CLIMBER. So I thought I'd make him something he can climb in different ways. Other parameters I considered when designing were:

Can be used outside
Foldable and not too heavy, so it's easy to carry around in the backyard/frontyard and to store for winter
Minimizing the number of wood cuts by using standard dimensions of timber

Instructions below are fairly detailed in an attempt to make this project accessible.

Supplies

WOOD
Note on type of wood: I used brown treated wood only, as I wanted it to be used outside and didn't want to spend hours and hours sanding and varnishing. Basic spruce would totally work, you'll just have to be careful about splinters and finishing; you'd be saving on money for sure, though!

3 pieces of 2"x4"x8'

8 pieces of 1"x6"x4'
or
6 pieces of 1"x6"x4' + 2 pieces of 1"x4"x4'

7 pieces of 2"x2"x4'
(I actually used bevel-end balusters, as it's the only treated 2x2 they had in store, so make sure you check all aisles in your wood store before giving up on it)

OTHER MATERIAL

3 hinges (3"x3")
1/4" Braided Polypropylene Rope (under 6' total)
Deck/Wood Screws:
- A lot of 2 1/2"
- About 18 screws of 1 1/2"
Wood Glue (optional)

TOOLS

Miter Saw (although a hand saw would be doable)
Drill
Drill bits:
- Drill bits to pre-drill holes
- Countersink bit
- Wood drill bits: 3/8" and 5/8"

Measuring Tape
Pencil (to take marks when cutting)
Lighter (or anything to slightly burn the rope)

For the option with only 1"x6" pieces of wood:

Table Saw

For the option when keeping the bevel-ends (see Step ):

Wood Chisel + Mallet
Flat Rasp
Hand saw

OTHER TOOLS

Your protective gear: glasses, earmuffs/ear plugs, etc.

Step 1: Visualize

Look at this Instructable a couple of times, review my drawings, do your own drawing, etc. In short: start visualizing and getting used to what you're going to build and what tools and material you'll need. I find there's nothing more frustrating than getting stuck because I don't have a specific tool or I need to go buy more wood.

You can check out my initial design in this SketchUp model:

https://3dwarehouse.sketchup.com/model/d3ef18c1-58...

You'll notice that I first thought of using round bars on the "back" side of the module. However, I ended up using 2"x2" pieces, as round bars were just too expensive for my desired budget (even broom handles were not as cheap as the 2"x2" I found).

Step 2: Build the 2 Frames

Material & Tools

3 beams of 2"x4"x8'
Miter saw (or handsaw)

As shown on the "SIDE VIEW" in the drawing, the length of the 2x4's for the sides is 46 3/4", and the angle is obtained by adding 1 1/4", requiring 48" (or 4') of wood for each side. The resulting angle is somewhere between 19 and 20 degrees, which was useful to know so I could check if it was an appropriate angle for a toddler to climb, however you don't need to know that exact angle to cut the actual pieces. My drawing is hopefully self-explanatory to allow you to draw the cutting marks.

That means you'll be using 2 of the 2x4 pieces for the sides, and 1 piece for the top part of the frames.

Cuts to make at this step:

Using the miter saw (or a hand saw, knowing you'll have a bit less precision for the angles):

Sides of the frames: 4 pieces at 48" (46 3/4" + 1 1/4" angled ends)
Tops of the frames: 2 pieces at 45"

Notes:

I used a couple of scrap pieces of wood to try and find the right angle on the miter saw, based on the 1 1/4" x 3 1/2" (i.e. the width of the 2x4) triangle.
Make sure you keep the angle as-is on the miter saw. You'll reuse that exact same angle later on.

Assembly:

Align a 45" piece parallel with an end of the angled pieces, the edge aligned on the "long" side of the angle (i.e. the "pointy" edge that's close to the floor in the picture)(picture #2).
If you don't align the 45" piece with the pointy edge of the angle, you'll end up with a gap between the frames at the top of your final module, also known as a "toddler fingers trap"...
You can use 2 or 3 screws to attach the 45" piece to the side beam, screwing in from the outer face of the side beam.
For all the holes (that applies to all holes for the rest of the Instructable...), I suggest:
- Pre-drill & Countersink
- Put some glue (optional)
- Screw in, using 2 1/2" screws
Repeat for the second side of that first frame, i.e. at the other end of the 45" piece.
Repeat all of what's above for the second frame.

Note: you'll notice that the bottom pieces are not added yet. If you want to, you can skip to Step 5 now so you can
finish the frames already, and come back to Steps 3 and 4 afterward. I chose to wait to have completed Steps 3 and 4 before 'getting to the bottom' of frames, as I wanted to make sure I had decided on the positions of the lowest 1"x6" board (on the "front" frame; Step 3) and the lowest 2"x2" beam (on the "back" frame; Step 4) before positioning the frame bottom pieces (Step 5).

Step 3: Add the 1"x6" Boards to the 'front' Frame of the Module

Material & Tools

6 boards of 1"x6"
One of the frames built in Step 2

Cuts:

Nothing

Assembly on one of the frames from Step #2

Start with the lowest board on the frame, as it will secure the width of your frame. Align the bottom of the board on the extreme edge (i.e. the 'pointy edge') of the frame sides.
- Make sure you're positioning the board on the "outside" of the final module. To make sure you're doing this right, you can make the frame rest on a wall, at a 20-ish degrees angle: you want the top piece to be flat (i.e. horizontal). If the top piece is not horizontal when you rest the frame slightly angled on a wall, that means you're looking at the "inside" of the final module.
Pre-drilling and countersinking are of special importance here, as you're working at the end of boards.
If you have some, putting some wood glue is not a bad idea here as it can help prevent the boards from splitting or twsiting on the long-term.
Attach the first board to both sides.
Attach each subsequent board leaving a 3" gap between each board.
- That 3" gap was a design choice in my original design. I wondered what gap my kid would need so he could put his feet in, including when wearing his mid-season boots. Even though I was already planning on installing climbing holds, I thought those gaps could add some safety to the module by providing an easy option for my kid to put his foot in. Also, I wanted to minimize the weight of the overall module to make it easy to carry.
- If you don't want a gap between the boards, or if you want to make it as small as possible, you can simply add a couple of extra 1"x6" boards on the frame.
For the last board, make sure you're aligned with the extreme edge of the frame. Here again you don't want toddler fingers to get stuck in that small gap...

Note: in picture #3, you'll notice a c-clamp on one of the board. The wood I got had some cracks in it, so I glued it back together.

Pallet?

At this point, you may be wondering: "What about using some good old pallets for those frames?". Indeed, that could actually be a viable solution, although it could end up being less solid, depending on the quality of the pallets you have access to (if any!), and less durable if you keep it outside. The only big difference would be that you wouldn't have a flat top to the module. My kid ended up loving sitting on that flat top and just staying there, sometimes gathering rocks or whatever, but that's definitely not a structural requirement. You'd also not have those flat "feet", which may or may not be an issue depending on where you'll end up using the module (grass, concrete, etc.).

Step 4: Add 5 of the 2"x2" Balusters to the 'back' Frame

Material & Tools

5 beams of 2"x2" bevel-end balusters
The second frames built in Step 2
If Option 2 (see below): handsaw, chisel, mallet, rasp

I went with 5 pieces of 2"x2" as 'steps' on the 'back' frame. You can choose to add or remove any number of steps you want, based on what looks good for your kids' size.

Here, if you're using bevel-end balusters like me, you have to make a choice regarding the way the 2x2's will be fixed to the frame. I've labeled them "Option 1", "Option 2" and "Option 3" on the attached drawing (picture #2) and below. Let's quickly discuss each of them and why I went with Option 2.

Option 1:

It's the easiest option to go with: on the frame, you simply put the balusters with the bevel facing up, and you predrill-countersink-screw it in. Pros: no cutting necessary. Cons: the 2x2's will stand out from the frame. I don't think this would cause any security issue, it'll simply make the module 2" deeper when storing and moving it around.

Cuts: none. Assembly (with frame laying on the floor): 1 vertical screw per beam end.

Option 2 (see pictures #3-4-5):

That's the one requiring the most work: you 'dig out' angled chunks of wood, and insert the bevel ends in those 'angled pockets'. Pros: who doesn't like using a wood chisel when one can? Cons: it took me about one hour to make the 10 'angled pockets' and adjust them with a rasp. Add to that the increased risk of hand-injury that comes with using a chisel and a rasp, and the many defaults caused by the imperfect measurements of cutting things at angles by hand (more on that in Step 6).
Honestly, I went with that option because of pride: I liked the look it gives to the 2x2's being 'inserted' in the frame, and the added complexity from which I can be proud of. I'd only suggest this option to builders who know how to use a chisel and don't mind the extra time it takes.

Cuts: angled pockets -- handsaw for the sides of the pocket (not pictured), chisel + mallet to take off the chunks of wood (picture #4), and finally rasp to 'sand' the side and bottom surfaces of the pockets and to adjust the width of each pocket to make the beams fit in (picture #5). Assembly(with frame laying on the floor): I had to hammer the beams in the pockets, and then fix them with 1 vertical screw per beam end.

Option 3:

By cutting off the two bevel ends from the 2x2's and getting to 45" long beams, you can insert the 2x2's at the depth you want within the frame. Pros: the 2x2's can arrive flush with the frame sides, which gives it a nice look. If not flush, whichever depth you put them at, you can arrange for them to be all at the same depth, which will make things easier in Step 6. Minor cutting required. Cons: if using 2 screws to secure the beam in place (see my suggestion below), that may be too much metal in the crosssection of the 2x2's. Other than that, it looks like a pretty good option I honestly wish I'd thought of in the moment!

Cuts: 45" long 2x2 beams (flat ends). Assembly: I would suggest using 2 horizontal screws per beam end (see drawing), to prevent the beam from rotating when stepping on it.

Step 5: Add the Bottom Pieces to the Frames

Material & Tools

2 pieces of 1"x6" + Table saw
OR
2 pieces of 1"x4"
Miter saw (or handsaw)

Cuts

Before cutting, make sure you measure the actual dimension between the two sides, at the bottom of the frame. This should be 45" in theory, however I had to chop a little more off for some reason.

If you were able to buy two pieces of 1"x4"x4':

Using a miter saw (or handsaw): two 45" long 1x4 boards.

If you were only able to get 1"x6"x4' pieces:

Using a miter saw (or handsaw): two 45" long 1x6 boards.
Using a table saw, cut the two 1"x6" boards on their length. I cut about 1 1/2" off the width, based on the space left at the bottom of the frame.

Assembly:

Insert a board at the bottom of one of the frames.
- Predrilling and countersinking are crucial here, as you're working both at the end of 2x4 beams and in really thin 1" thick boards.
- Wood Glue (optional)
- Screw in, using 2 1/2" screws
Repeat for 2nd frame

Note: on the 3rd picture, you'll notice that one of my 2x4 frame sides got bended. You can see the resulting gap with the bottom piece at the forefront of the picture. That bending happened because of the hammering I did to get the 2x2's in the 'angled pockets' in Step 4.

Step 6: Add the Inner Supports

Material & Tools

2 beams of 2"x2"x48"
Miter saw (or handsaw)

Those inner supports are meant to prevent the 'horizontal pieces' on each frame (i.e. the 1"x6" boards on the 'front' frame and the 2"x2" beams on the 'back' frame) from bending and (maybe?) eventually breaking under the load of kids climbing back & forth on them.

Cuts

'Inside' one of the frames, measure the distance between the top and the bottom pieces of the frame.
Measure again, as you're never too sure.
Using the miter saw at the same angle than the 2x4's in Step 2 (or the handsaw), cut the 2x2 beam at the required length.
Repeat for the other inner support (the length of this second 2x2 beam may be different than on the other frame. At this stage, the frames have enough imperfections that you should rely on actual measurements rather than on theoretical ones).

Assembly

Position the 2x2's at the center of each frame, inside it, touching the 'horizontal pieces'.
- On my 'back' frame, remember how I went with Option 2 in Step 4 and hammered the beams in place? Well, those beams were therefore not all at the same depth, so the inner support was actually not touching them all when I installed it. The gap was bad enough for the top beam, that I inserted a piece of scrap wood so I wouldn't have to bend that beam too much to screw it to the inner support.
Use 2" or 2 1/2" screws to assemble each 'horizontal piece' to the inner supports.
- Predrilling and countersinking recommended
- Wood Glue is not relevant here, as the inner supports are not that wide, and the screws holding them are fairly close to each other, so the risk of the beams bending or twisting over time is minimal.
Use 2 1/2" screws to assemble the top and bottom pieces of the frames to the inner supports (the screws coming in from the outside, longitudinally toward and in the inner supports).

Note: On the picture, red arrows are indicating the inner supports. I took that picture after I had finished the whole module, so hopefully it's still easy to see what I did here.

Step 7: Assemble the Two Frames!

Material & Tools

The two frames you just completed in the previous step
3 hinges(3"x3")
- About 18 wood screws of 1 1/2"

Here comes the time when you feel like you've actually built something! You're going to assemble the two frames in that reversed V-shape.

Cuts

None

Assembly

Flip the frames upside-down, bring them to contact in a V-shape (picture 1), and use boards of scrap wood to hold each frame in place (picture 2).
- Slightly separate the two frames away from each other, trying to keep them as parallel as you can, leaving a 'slight gap' of no more than 1/16". This little gap will ensure your two frames don't bump into each other when you open the module.
Position the 3 hinges
- For the hinges at the extreme ends, I kept about 1" between the hinges and the inner sides of the frames (picture 3).
- For the center hinge, I couldn't fit it exactly in the center as one of the inner support was in the way, so I moved it just past it (it doesn't matter if it's on the right or on the left of that inner support).
Mark the positions of the screws
Predrill and add the screws (no need to countersink here, as the screw heads will rest on the hinges themselves)
- When screwing in, make sure the hinges stay as parallel as possible to the edge of the frame. You want the axes of the hinges to be all aligned together AND all aligned in the center of that 'slight gap' you created between the two frames. The more those things are aligned, the less the wood of the module will be bending and twisting over time.

Step 8: Add the Rope

Material & Tools

1/4" polypropylene braided rope(under 6' total)
Scissors
Lighter
Wood Drillbits: 3/8" and 5/8"

The rope is an extra safety to keep the module straigth when it's open. Without it, only the hinges would be supporting the weight of kids climbing in and off the module, and you wouldn't be able to maintain the top of the module flat, i.e. the top pieces of the frames would eventually bump in each other. Overtime, all those constraints would eventually damage the wood.

Cuts

Cut two pieces of rope of about 3' each

Assembly

Locate where you will drill the holes for the ropes
1. Put the module on its feet, and close it as much as you can. Your two frames should touch at the bottom. Pinching a piece of rope, start from the bottom and, going up, locate how high on the frame you can first insert the pinched rope (i.e. where you can insert two pieces of rope at once) (picture #1). Finding that location will make sure you're not putting the rope too low, which would make the rope 'bump into itself' and pinch itself everytime you'll close the module.
2. Open the module, and try having the flattest top you can (picture #2). This will be the position of your open module for the rest of its useful life.
3. Around the height you've identified to locate the rope, use a measuring tape to find the best possible location that fits both sides of the module. As you can see in picture #3, I didn't have many options: anything below 24" (as measured from the bottom of the frame) would have been in the 2x2 beam on the 'back' frame, and anything above 25" would have been in the 1x6 board on the 'front' frame. Knowing I eventually needed a 5/8" wide hole, I chose 24 1/2".
Drill the holes
1. Using a 3/8" wood drillbit, drill a horizontal hole across all the sides of the frames (pictures #4 and #5).
2. Using a 5/8" wood drillbit, drill a horizontal hole about 5/8" deep (picture #6). You're looking to do what is called a 'counterbore'. Honestly, I didn't measure that depth in the moment. I just drilled deep enough for a knot in the rope to fully fit in that hole.
Attach the ropes
1. Insert a rope in one of the holes, and make a simple knot at this end. Check if the knot fits into the 5/8" 'counterbore' hole.
2. Insert the other end of that rope straight in the facing hole.
3. After making sure your module is open at the exact angle at which you want it, pull hard on the other end of the rope and use a pencil to identify where you want the second knot to end (picture #7).
4. Slightly close your module to have enough loose in the rope to comfortably make the second knot at the penciled mark.
5. Repeat for the second rope.
6. Using scissors, cut the extra lengths of rope beyond the knots.
7. Using a lighter, burn all the knots for a half a minute or so, enough for the plastic fibres to melt together and secure the knots in place (picture #8).

Note: Props to this Instructable for the idea of the polypropylene rope and the burnt knots.

Step 9: Finish It As You Want

Material & Tools

Climbing Holds
Wood drillbit (diameter based on the screws coming with the holds)
Outdoor Varnish (optional)

We'd found climbing holds on Facebook Marketplace for cheap, so that's pretty much what started this project of building something to put the holds on. If you don't have holds, or at least you haven't found affordable ones yet, the module can already be used as-is!

Cuts

None

Assembly

Have fun positioning the holds to make it just hard enough for your kid :)
A good principle you can use is to think of the centre of gravity of the child: when putting their hands there and there, where will their centre of gravity be, approximately? You can then put a foot hold right below that centre, at the right distance for your kid(s)' size, or anywhere to the left or to the right to make it a little harder to climb from that foot hold.
The climbing holds I had were coming with bolts and nuts (tee nuts to be specific), so I drilled holes that were slightly larger than the bolts, but still smaller than the nuts.
The kit was also coming with an hex key for the bolts, so assembling was pretty easy there.

Wood Finish (optional)

Because I'll use this outside, I put a final touch of outdoor varnish on the parts of the wood that were not treated, i.e. everywhere I had made a cut and I could see untreated wood. Mostly, I was worried about the 'feet' of the frame, as I may put this module on grass for a couple of days at a time, and therefore the untreated feet would have been in direct contact with the sometimes damp soil below.

Congrats, you just made a folding climbing module!!

Safety note: never leave an unsupervised child use this module. If you can, and especially on harder grounds (i.e. concrete, cobblestone, etc.), place slim mattresses around the module to dampen any inevitable fall. Believe me... it's fun for no one when you have to soothe a toddler who fell on his back on concrete from even just the second step, i.e. not even 1 foot high!