Everyone hit the Civil War attractions, so I will offer some outside ideas.
Like someone said, Lancaster isn't too far away. Its a beautiful town and the surrounding Amish communities are great to visit. Lots of little shops and antiques, great food and quaint Amish farms.
Hershey isn't very far away either. If you have kids (or are kids yourself) Hershey Park is a great amusement and water park. They have the Hershey plant with tours too. I've never down the Hershey plant but hear that it is fun.