I took my daughter for a 3 day trip to DC. It was great.
The first day (after a 6 hour drive), we went to the zoo. I was kind of disappointed in the zoo. My childhood memories had it much nicer than it was when we were there. However, it was a few years ago, and it was a bit under construction, so it might be much nicer now.
The second day, we took the DC trolley tour, which is a hop on hop off tour.
http://www.trolleytours.com/washington-dc/
The driver was very informative, and it was a lot better than just seeing things on our own. Plus, it got us up and down the mall very quickly.
We also took advantage of their monuments at night tour, which I would also recommend.
Of course, there are all the museums. We had a wonderful time in the national gallery, and the sculpture garden. Also, the Air and Space museum. All the Smithsoneans are wonderful, so you just have to see what you're interested in. I would not recommend having lunch at the Air and Space museum, not much to choose from, but there is a very nice cafeteria at the Natural History museum I believe. Also near the Natural History museum is the Old Post Office, which has the clock tower (free to go up) with wonderful views of the city. Ben & Jerry's at the Post Office is a nice treat.
Union station was a good place to go with a group for a quick meal, as there are lots of different food vendors to please even the pickiest traveler.
I would highly recommend the trolley tours as it's a very inexpensive way to get around, and also get a bit of a tour, with someone telling you things about the sights that you would otherwise not know.