In order, I learned how to put up a hammock made out of just about anything.
I learned of the monkey fist ball knot. They say to put something in the core, but mine doesn't have anything in it. It just is. It isn't pretty, but it functions. I would have liked to known about that when I was scuba diving and had to pass lines between boats.
I used my 100 foot repelling rope (that I've had since I was about 18) to make the monkey fist. Problem was I had it wrapped pretty tight. In making the fist, I ended up with a mess of rope on the floor. So then I went and learned how to do a backpack coil, and a mountaineering coil.
Then I finally, after watching about 30 minutes of youtube videos, figured out how to pick a pad lock. I've had a Master M1 (it's older than I am) that I've been trying to pick for about 10 years or so. I would get discouraged easy, and then I lost my picks for a long time. Found them a couple of months ago, but still got discouraged easy. Tonight, I've picked the pad lock 6 times already. still want a new / better set.
Then looking at the coil of rope, I undid it again. Put a monkey fist on the end, and then recoiled it.