Cameras are my biggest weakness. I collect the whole gamut, from 8x10s to micros. I got into collecting 110 SLRs & accesories for a while... they don't take up too much space

And vintage film canisters. I've gotten lucky a few times picking up various glass/optics (including microscope elements) insanely cheap and flipping. makes me feel not quite so bad about it all.
It's sort of bicycle related (but almost toy-ish), but I've gotten into vintage European (specifically Soviet) circus & childrens bikes:
In terms of non-collecting, cooking is probably my biggest "hobby" if you will, although that has led to other collecting sicnkesses, including vintage mason jars (I make my own mustards and preserves), Fiesta Ware, and vintage glassware.