Washington DC is the heartbeat of the entire United States nation. Attractions here aren't hard to come by, it's getting it all into one trip that's the problem. Stop by the National Mall and see the Declaration of Independence at the National Archives. Sit at the steps of the Lincoln Memorial. Walk along the Vietnam Veterans Memorial. Enjoy all the US Capitol has to offer.
If you're going to be doing some traveling to finish off the summer, check out DC. There are some pretty good deals for your vacation with your family or with a friend. Find great hotels nearby the capitol or many of the attractions.