Things to do, see in Philly?

My niece will be graduating in May from a college near Philadelphia. Since we’ll already be going out there for her graduation, we’d like to take a few more days (5 to 7) and play tourist. What are some “must-sees” and “must-dos” around Philly?

I lived back east many, many years ago, and I remember Pennsylvania Dutch country, the Liberty Bell, Betsy Ross house, Gettysburg, and that’s about it. What else do you recommend?

I’ve never actually had a real Philly cheesesteak, so I suppose that should be in our plans. :-)