Great thread!!!
My first impulse is to say football, but that was too easy and too similar to collecting baseball.
It would have to be non-paper items, so no comic books (which I do like)etc. I like things I can display, that have visual appeal, are rich with nostalgia, and that at least some people can relate to.
I could easily see myself collecting brewerianna, or old advertising trays.
Here is a KILLER collector site. I could get into these soooo easily.
http://www.trayman.net