I live in the DC area, and agree with what has been posted. Here are a few more things I'd like to add.
The basement of the Ronald Reagan building has a huge food court, and it is just up the block from the American History museum on the Mall.
If you want to see the museums, monuments, Capitol Hill, and other places and don't want to take Metro, a cab, walk, or drive, do the Tourmobile.
http://www.tourmobile.com/ It goes everywhere, including through National Cemetery. I don't know what the prices are for the different services, but it is worth the $$ not to have to figure out how to get anywhere!
The new Udvar-Hazy Air and Space Museum Annex
http://www.nasm.si.edu/museum/udvarhazy/ is a good day trip if you're really into airplanes, etc. You can take a shuttle from the Air & Space Museum on the mall for a fee. My only warning is that if you have a fear of heights, beware. The only way to see many of the planes "up close and personal" is via floating walkways. Buy it is amazing to be in a building that dwarfs the size of the Space Shuttle!
During the summer, usually between Memorial Day & Labor Day, the various military branches will host free outdoor performances, scattered throughout the downtown/mall area. My favorite is the Marine Corp. Drum & Bugle / silent drill team at the Iwo Jima memorial. You can find out what's going on once you arrive in the area by either checking the newspaper (Friday's Post includes the Weekend section that tells what's going on for the next week) or just asking around.
And don't forget the Washington Nationals baseball! Go Nats!