OK, first, go easy on me, this is my first instructable. Second, this is a working prototype. After a couple of outings, I have a few ideas on improving it (I'll note those in the steps where the changes would be made).

Let me give you the situation, and how I came up with my solution. I have a 2 year old, 60 lb, Catahoula/Lab (Maybe some pit??) mix. He's lean muscle. He loves to go for long walks. I'm planning on taking him on hiking trips. And, he's my best friend.

Most leashes are just not versatile enough for my tastes. They are either short, and sturdy, or retractable and fragile. Few give you options on how to hold or even to attach them (Tether) when needed. When you are on the trail, you need options. LOTS of options. There are times you want to let them have extra line to explore. There I times you need to keep them close. Sometimes you need to be hands free and attach them to your waist. Or, you need to attach them to a tree/post while taking a break or making camp for the nite. Seriously, you could spend over $100 on an assortment of different leashes for each task (retractable/extendable, waist, tie out, control....). I built this working prototype that handles ALL those needs, and then some, for under $10 worth of materials!

Step 1: Materials/Tools List

Here's what I used. Total, I spent more than the $10 mentioned above, but I only used a small fraction of the rope and twine. The remainder has been added to my supplies for future projects. I'll put the total costs, as well as the estimated costs in the list. You may even have some of this on hand.

I picked the materials based on my dog and my needs. If you have a smaller, lighter dog, you might consider using smaller rope and clips.

MATERIALS: (All of this was bought on the same isle at Home Depot.)

3/8" Diamond-Braid Poly Rope (100 ft for $10, I only used about 20ft, so about $2 worth) - I chose this fora few reasons. Cost, it's soft on your hands, strength (244lb working load!), It floats, and it's mildew resistant (You will be using it outside. There will be rain, they will get into water....)
Twisted Poly Twine (250 ft for $3, I only used a few feet, so less than $0.50 worth!). - This is only used for securing the ends of the rope and making it look nicer. If you wanted to be cheap about it, you could just use duct tape. I wanted it to look better.
5/8" Swivel Eye Snap ($3) - I went with the 5/8" because it has a working load of 130 lbs, Henny is 60 lbs of lean, hyper, muscle. I wanted to make sure it would hold up.
2 Carabiner type clips. ($1 each) - These have a max load of 150 lbs. Not for climbing, but more than enough for our pup.

TOOLS:

Scissors or knife to cut the rope/twine
Lighter to fuse the end of the rope
Sewing needle with an eye large enough to thread the twine
Needle Nose pliers (Not pictured because I didn't realize I needed them, until I needed them!)

Step 2: Start at the Clip End....

I didn't measure out my rope and cut it off the roll. Honestly, I wasn't sure how much I needed. So, I just fed it off and worked with it, then cut it at the end. So, there are no full measurements here. Those with OCD might have a heart attack going forward. But, honestly, I think much of this is going to vary based on your dog, your preferences, and your needs.

Start with a very simple larks head/Lanyard hitch knot (Also known as a "Cow Hitch" or "Reverse Double Half Hitch") to attach the rope to the swivel clip. Make sure you have about 6" of free line left over.

NOTE: this is one of those places you could use duct tape, or friction (Electrical) tape to wrap if you wanted to be quick and easy. I wanted it to be more durable and look better so I used thread.

Spool off about 5ft of twine and thread it on the needle. Start by stitching it THROUGH both the line and the lose end and tie it off. This will hold them together while you work. (This is where I found out I needed the needle nose pliers to pull the needle through the rope.)

Wrap the twine around both bits of rope until you have about 8" left. Be sure to pull this tight as you go so it bites into the rope to hold it. If not, the hitch knot can fail as it will eventually pull the free end of the line down. By binding it like this, you will keep it from being able to pull back through.

I threaded the end of the twine back onto the needle to push it down through the wrap (Again, the needle nose pliers came into play). I then tied it off at the end, trying to make it look nice.

Step 3: Making the Loops for Expanding the Leash.

Come up about 18" from the clip and tie a very simple Loop knot. I used a basic overhand loop. Make sure this loop is large enough to easily put clip the carabiner in and out of as this is how you will shorten the leash. This will also be my grab point for when I need to hold him close (In crowds, around other dogs.. anywhere I need a close, control hold on the leash...). You may need to make this longer if you have a shorter dog, or shorter if you have a taller dog... and based on how you like to control your dog.

Then, loop off about 18" to 24" of rope (Again, your preference based on how much extra length you want with each section). Tie another overhand loop. Clip this to the first overhand loop with the carabiner.

Repeat this step as many times as you want to add expandable sections. I made 3 expandable sections in mine. I think 4 is workable. But, the more you add, the heavier the leash will be on you and your pup.

POSSIBLE MODIFICATION/IMPROVEMENT:
OK, one thing I noticed after using this was that the loops of rope can drag the ground and can be a little bit in the way (or even snag). Most of the time, that was not an issue. Henny kept enough tension on the line they stayed up off the ground.

When the loops are all taken up, the load is all on the two end knots and the carabiner. The other knots are all just hanging the loops from the carabiner.
I'm thinking of cutting these loops of rope out (Being sure to tidy the ends of the line with the twine like at the clip). Then, replacing them with shorter loops of Bungee cord. This would allow for less material (Smaller loops, less weight), while still being able to expand when I unclip the carabiner and want to let him explore. Please post your thoughts on this in the comments.

Step 4:

Once you've added all the expansion loops you want, it's time to finish off with your grip loop. I measured up 5ft from the last knot to be the start of my end loop. I spooled off about another 24" of rope, cut it off the roll, then fused the end with a lighter.

I folded this back over itself and made another overhand loop knot at my 5ft mark, making a very large loop. I then came up about 3" and made another overhand loop knot leaving a smaller loop on the end with a 3" section of double rope between the two loops. This is where I put the other carabiner. This gives me a place to loop the end around a pole, bench leg, tree, whatever to secure him as a tie out. I also clip my water bottle, and Henny's water cup there. (See the last picture). I can even wrap the line around my waist and use the carabiner to make it a hands free leash.

Finish it off by binding the loose end of the rope below your knots with the twine like we did down at the clip.