When I travel I like to check out the used bookstores in whatever city/town I'm in. Besides the fact that they're all unique and worth checking out for their own sake, it's a way to explore neighborhoods you otherwise would have no reason to visit. Often the selection of books will reflect specific things about the community, too, which makes sense since the inventory is largely stuff locals have sold to the store. I remember a store in Bozeman that had a whole section of fiction by authors from Montana (and/or set in Montana). Good stuff, mostly, too.

You can find great stores in college towns too. Boulder, Berkeley, Bloomington, Burlington... probably some cities that don't begin with "B"... One that sticks out in memory was in Lawrence, Kansas. Can't recall the name, but a small, cute store with an interesting collection and good prices. I try to keep bookmarks from the store with whatever I purchased there.