Driving up the coast of Maine, it seems like you pass a used bookstore every few miles. The ultimate is the Big Chicken Barn; I could spend a whole day in there. If you live anywhere in eastern New England, a trip up Route 1, perhaps with one or more of the used booksellers' associations' store lists in hand, will be worth a day or two. (I'd recommend a weekend trip, one day up and one day back, because some of the best close early). Oh, don't stop at the one right over the bridge -- I forget the name, but it's a freestanding little place on your left with a shabby parking lot and piles of rotting books out front -- the owner demands a browsing fee (and there are some creepy rumors about him).
I miss those trips.