No firearms, no ammo, no brass, no expended rounds, no .50 cal bottle openers...
Beyond that, Arlington National Cemetery, Changing of the guards, if you can find one where they are doing a wreath laying, much more impressive. National Air & Space museum, and the ANNEX to the museum, it's a ride, but the thing is packed wall to wall with planes to include an SR-71 and one of the Shuttles. Walk the National Mall, especially if there's good weather.