If you had one day left in Washington, DC and already went to all the museums, etc., where would you go?

Just finished my internship in Washington, DC. I have the weekend to myself. So far I have seen all of the monuments and museums as well as done most of the tours including the White House, Pentagon, and the Capitol (lol OMG did I do the Capitol). Where should I go to spend my last weekend here? No car stops, within a 30-min walk of Metrorail stops, please.