I hate big cities, but love Washington DC. Sounds like you will hit up all the local goods. I have been to Washington DC 3 times and highly enjoyed each time. Everyone says you have to take the subway. I don't think people from the east are like people from the midwest. We walked everywhere and it wasnt as bad as everyone made it out to be.
1 off the beaten path that others probably haven't recommended. Take a cab to the Watergate Hotel. Ask the "bell hop" for the Nixon tour. It is free upon request (you will want to tip the guy but he does a fantastic historical tour) and you see everything how and where it went down. If you have kids he'll also load them up on free souveniers. It was one of the cooler more personalized tours of Washington DC.
The theater where Lincoln was assassinated in was also a really good tour. That is only a few blocks away from the White House and a very easy walk.