Philly's a very artsy town:
Philadelphia Museum of Art, (where Sylvester Stallone famously ran up and down the stairs) in Rocky,
Pennsylvania Academy of Fine Arts, and
The Rodin Museum, to name a few faves.
The
Mutter Museum is also in Philly, if you're into that sort of thing.
And there's always the Philly Cheese Steak dilemma: Trying to figure out who makes better cheese steak sandwiches - Geno's or Pat's. It's a highly divisive issue in Philadelphia.
Oh yeah, and you might want to drop by and see that Liberty Bell thingy while you're there.