Often while we are on vacation we will explore antique shops and antique malls. Personally I prefer the antique malls since they usually have a lot more stuff, and a larger variety of antiques.
Usually I will roam the aisles looking for used books. If something else catches my eye, I will stop and look. My wife will wander up and down the aisles separately looking for whatever appeals to her.
For the last day we have been staying in Shoreline, WA. There seem to be no used bookstores here, but there are several antique malls. We just got back from a large one with over 250 vendors. I combed the place looking for vendors selling books. There were quite a few books there. My main interests are martial arts books, but I rarely find any of these in an antique store or mall. But I also collect modern fiction, and will sometimes find a cool under-priced book. I did find a copy of a book that my wife wanted to read (nothing collectible).
You never know what you are going to find. I found a gumball machine for $25. If I found it near home, I would have bought it. But trying to transport it back to New York would be a pain.