There are so many wonderful things to do and see for free in DC!
Don't miss the National Zoo. Check the calendar of events at
www.fonz.org
The National Geographic has a great museum and it usually isn't as crowded as the Mall.
http://www.nationalgeographic.com/museum/
The National Museum of Health and Medicine is very cool:
http://en.wikipedia.org/wiki/National_Museum_of_Health_and_Medicine
The International Spy Museum in DC is supposed to be really cool.
http://www.spymuseum.org/
And, if you're REALLY lucky, you might get to see the Cherry Blossoms!
http://www.nationalcherryblossomfestival.org/cms/index.php?id=390
The Cherry Blossom Parade is slated for April 12th.