We just did DC last week for Spring Break. We had a great time. It was nice to do something different than sitting on a beach or skiing. I just read GOLL's post above and damned if we didn't have almost the same trip.
Hotel - The Mayflower. We loved it. We got a suite with a room so our son slept on the fold out couch in the living room. The room also had two bathrooms (only one shower).
Museums/Monuments - Air & Space, Natural History, Spy, Holocaust and Ford's Theater. We also walked The Mall and saw all of the monuments. We also walked around Arlington and saw the Changing of the Guard which is always awesome. One night, my wife and I went back and did the monuments again. They really have a different vibe at night. I actually enjoy them more.
Food - Old Ebbitt Grill was amazing. I could eat there every day and be fine. Filomena in Georgetown was great as well. We did Ben's Chili Bowl one night. Founding Farmers was pretty solid as well.
We had a great trip and I'm really glad we did it. We love the city. I actually feel like it would be a great place to live if you could afford it.