I agree with Reading Terminal Market, make sure you get a cheesesteak from there, awesome. Sorry people, born and raised in the Philly area, and IMHO there are much better places than Pat's and Ginos to get a cheesesteak. I prefer Jim's on South Street. That's another place you could visit, South Street is very eccentric, but cool. There is a pizza place on 2nd and South, I think it's Delorenzo's, the pizza slices are huge about as big as my arm, and delicous!
I still prefer the Art Museum, I had to go there to do a paper for my college Cultural Anthropology class, some really beautiful exhibits! Also, the art museum is the steps where Sylvester Stallone ran up them in the Rocky movie, also has a really pretty view of the skyline.
Independance Hall too of course, how can you not go there when you come here to visit!
I hope you have a wonderful time when you come here to visit!!!