Since you've been to DC, then that was one of things I will not suggest. We never made it to DC with our kids, and that is one big regret of mine.
I would suggest, if you are in the NE area, do the Pilgrim thing! Plymouth Village and the ship Mayflower II in Massachusetts is great to visit. Note about Plymouth Rock: it is, literally, just a rock - so ask a nearby Park Ranger for info about it. They have a really good 15ish minute historical talk about it!
Also, my kids, although young (pre-teen I think) did think Gettysburg Battlefield National Park was cool. The park is huge, so everyone gets a CD for their car and does a "self-guided" tour. It works out great and allows you to do things at your own pace. We stayed at Artillery Ridge campground, which has corrals and stalls for horses (just in case you want to bring yours), so it was neat with horses in campground.
If you manage to get all the way up to upstate New York, Fort Ticonderoga at the northern end of Lake George is really cool.
In Connecticut there is, always, Mystic Seaport. A little further inland is Gillete Castle state park. Of course, every single town and green in CT has a historical story.
