If you want the true historic Philadelphia experience, you have to visit the Liberty Bell, Independence Hall, and the Betsy Ross House. I suggest taking one of the hop-on, hop-off bus tours so that you can see everything in a day. If you want a museum-oriented experience, visit the Art Museum (where you can run up the steps like Rocky Balboa), the Franklin Institute, and the Rodin Museum (largest Rodin collection outside of Paris). You also have to try the food that is unique to Philly - cheese steaks, water ice, and soft pretzels. There is a whole thread on FlyerTalk debating the best place to get a cheese steak (which isn't a steak, but rather thinly sliced beef with cheese on a long bread roll).
For a day drive, head out west to Lancaster County to visit Amish country. You could also go to the Hershey chocolate factory. A little further out will take you to Gettysburg, site of the one bloodiest battles in the Civil War. If you head east, you can spend the day "down the shore" as those of us in Philadelphia say when we go to the New Jersey beaches. New Jersey shore towns are beautiful, especially Cape May at the very southern tip of New Jersey. It has loads of Victorian era homes that have been restored, and many of them are bed and breakfasts. You can also stop in Atlantic City if you feel the need to gamble. I suggest going to the Borgata hotel and casino.
Hope this helps!