We went to Williamsburg a few years back now and found it to be a really good place for a slightly more relaxed American adventure. We travelled around a fair bit and visited all sorts of interesting places.
We did Busch Gardens (which was great btw), Colonial Williamsburg (the living history interpreters are really interesting to talk to). Other places to look into are Historic Jamestown, Carter's Grove (Where we met a fantastic interpreter called Mr Watson in the slave plantation huts),
I think that we drove from Washington to Williamsburg, stopping a few times on the way. If you can go further afield, and don't mind the driving I'd highly recommend heading up to Alexandria (just south of DC) and checking out the History and Archaeology Museum there. It's fairly close to Mount Vernon (George Washington's Home, which was undergoing major renovation when we were there) which is also worth a visit if you like your history.
Even further afield, we spent two days driving the Skyline drive, through the Shenandoah National Park, along the mountain tops. A really good place to go if you like spectacular scenery, and there's a couple of nice guesthouses up there to stay in. If you start at the top and work down you come out fairly close to a number of historic sites including Lexington, and about half way down the trail you can take a detour to a place called Luray where they have some really cool caverns.
Sorry if this all seems a bit confused, it was a while ago, and the year before that we went to Boston and travelled around there too, so I forget precisely what we did when!!! If you want any more info on any of the above just say and I'll dig it out. My family hoards travel docs and maps and stuff from every holiday we've ever been on!
Milia