You could spend the better part of day wandering around the BPL. It's a great experience on many levels. You can check out the Sargent murals, poke around special exhibits, wander through the big reading room (which reminds me very much of the main reading room at UC Berkeley), or just browse the books. Do watch your wallet/purse/bag since a lot of homeless people like to spend the day in here, too, especially in winter. Be aware that if you want to borrow a book and are not a resident of Boston, you will need to register first (you can borrow books with your card from another Minuteman Library, but you need to register in a separate room before check out). Now they have a nice cafe here, too. Check out their free films and lectures and other events.