One of my favorite things to do while shopping is to wander through a bookstore. Even better is if that store sells used books. Finding that hidden gem at a great price is as good as it gets.
Philadelphia has a large selection of bookstores, but my favorite is the Book Trader, located near 2nd and Market Street, not far from Independence Hall. The Book Trader has been open since 1975 and was originally located on South Street but moved to their current location in 2004. The shop is crammed with books and is spread out over two stories, with each type of book found in a particular section. In it, you will find anything from classic novels to current non-fiction. And for those of you that enjoy the old-school way of enjoying film and music, they also carry CDs, DVDs, and LPs.
If while in town you have a book you are done reading, take it to the Book Trader. They would be happy to trade if for store credit, giving you a new read for the flight home.