I'm in Washington fron Dec 24th to 30th. I'm gonna try and go to the Pentagon, the Lincoln Memorial, the White House, Arlington National Cemetary, National Air & Space Museum and the Capitol.
Forgot to mention that The Pentagon is just a stones throw away from Arlington National Cemetery but they are both across the Potomac from the other features mentioned...
I'd recommend a walk around the Tidal Basin and a visit to the Jefferson Memorial too - its my favourite of all the big memorials. Also make sure you see the Vietnam War Memorial and the Korean War Memorial - I find the Korean one particularly moving.