Learn how to tie various knots. This is useless but awesome.

Not so useless. I've gotten a surprising amount of mileage out of a Boy Scout-level knowledge of ropework; it's one of those skills that doesn't come up very often but makes your life vastly easier when it does. Tying a tent down in high winds, for example, is made much simpler if you're familiar with the bowline or one of its variations.