Harper's Ferry, W VA
Gettysburg, PA
Many years ago, my parents took us kids to Ft Legionaire - there was a fort set up like it was back in the day. I don't know if that's still there or not. That was out in western PA.
You might want to get them each a scrap book and let them keep a record of your adventures together. We took my GS on a long trip last summer. Left him make a scrap book -- he bought some postcards, picked up brochures for places we visited, and after we got home, I added photos of us to the appropriate pages. I had him write a few lines about a couple of things we did. It will be nice for them to look back on. He was 8 at the time, and really seemed to enjoy it.