Spy museum is at least a 4 hour adventure and worth it.
Holocaust museum is another 2 hours.
If you have time the changing of the guard at Arlington Cemetary is pretty cool. Good photo op with your XT. Interesting fact, as you come down the hill from the changing of the guard there is a HUGE tombstone with the last name of Booth. This is the brother of the assasin of Abraham Lincoln, John Wilkes Booth.
The Vietnam Wall is another photo op and the Korean War Memorial too. And the WWII memorial is there now right by the Lincoln Memorial. It was finished about 2 weeks after I moved from the DC area back out to the Southwest.
Take you good walking shoes and some water. You're going to need it.
And stop in front of the White House. It is pretty cool to look at.
What lens(es) are you taking? And make sure you have plenty of CF cards as they will fill up quickly.
The Smithsonian is cool and the Air & Space Museum is right there on the Mall. However, that is another 2-3 hours.
Enjoy DC.