Rope.

Buy some rope, like 3 different sizes: 1-2mm, 4-6mm and 10-12mm. Watch a few rope videos and then throw the ropes in some drawer. Every now and again you will have an issue that the ropes can fix.

As a kid I once got a 15mm*40m rope for my birthday from my uncle, I thought it was the weirdest thing, but over the years I used it so much that these days I consider it to be one of the best gifts I've ever received. And as long as you don't need them for anything related to safety, they basically cant expire.