There are tons of restaurants around. Pick something you haven't tried before. The Old Post Office has a food court that's better and cheaper than eating in the tourist traps.
Travel by Metro. You can get all the way from Springfield, VA to Germantown, MD if you take the right combo of buses and trains. If you drive, you will need a SmarTrip card to get out of the parking lot. Look at wmata.com and determine whether an unlimited pass (week or day) would best suit your needs or if you'd rather buy a SmarTrip card and just add money to it. Consider exploring the suburbs, too.
There are a lot of museums. Some are free (National Gallery of Art, Air and Space) and some are pay (Spy, Newseum).
If you get up to Baltimore, go to the Inner Harbor and the Aquarium. Consider a Duck Tour.
Do you like theatre? Check out the Kennedy Center. See if Sheer Madness is available. Great show.