The version I made is pretty sparse but you can add ladders, stairs, swings, pulleys, pretty much anything you want. Right now my son uses it for his peg people.
Step 1: Tools and Supplies
-A Sander (or sand paper and lots of elbow grease)
-A Drill with a Bit the same diameter as the dowel you intend to use
-A Large Board (or several smaller pieces)
-A Dowel Rod
One of the great things about this project is that you can make it from scraps. All of the wood in this project (including the dowels) I just happened to have around. You don't even need a lot and it doesn't even need to match.
Step 2: Cutting, Drilling, and Sanding the Pieces
First draw out and cut your board into the shapes you want. You don't even have to draw them first if you don't want to. Just go to town with your saw. Then cut your dowel rod into the lengths you want.
Tip: The shape is totally up to you, but consider avoiding sharp or pointy edges.
Drill the holes for your dowel rods. I tried to drill the holes deep enough to hold the dowels without going all the way through. Alternatively you could offset the dowel rod placement and just screw it together, but I didn't want any visible hardware.
At this point you can sand your surface pieces and they will be ready to go. I didn't bother sanding the dowel rod because it was already smooth. The bit that I cut was a little rough but it gets inserted into the holes so it doesn't matter.
Step 3: Gluing It Together
Tip: If you make a platform like the single level one I made on the right, your child may decide it looks like a seat. This may result in it coming off and you having to re-glue it back on in a different direction so that he doesn't continue to sit on it.
Step 4: Play!
At this point I haven't added any extras, but you can always let your child's play dictate what extras to make; they're easy enough to add later.