The possibilities are endless!
You could swing into the historic section and see the Liberty Bell, stop in to Besty Ross's house, and visit Independence Hall.
Here's a link for the historic sites: http://historicphiladelphia.org/#void
You could stop in at any of the museums in Philly. The Franklin Institute but if you go there you should check out the Tuttleman IMAX theater! Totally an amazing experience! The screen is a huge dome that surrounds you. The entire museum is an amazing experience that the whole family can enjoy!
Here's a link for the Franklin Institute: http://www2.fi.edu/
Another fantastic place to stop while in the Philly area is the Philadelphia Zoo, which is America's first zoo! There's something for everyone, just be sure to wear very comfortable shoes and sunscreen, this zoo is a full day event but you won't regret stopping in! Be sure to see the Big Cat exhibit as well as the Primate House!
Here's the link for the zoo: http://www.philadelphiazoo.org/
I hope you guys do stop in Philly, especially if you've never been there before! It's a very worthwhile stop with lots and lots of things to do and see!