Bottle caps. Hundreds of metal bottle caps. I started doing that something like twelve or thirteen years ago, and haven't really stopped, though I'm not doing it very actively anymore (and haven't ever really bought any, or bought a bottle for its cap). I always get a few from relatives at Christmas and such occasions, especially from one of my uncles, who is a musician/music journalist and gets around a lot, which apparently runs in the family.

Fuzzpilz