The Air and Space Museum is popular and one of my favorites. Someone mentioned the Holocaust Museum. Very riveting. You will walk out changed in some ways. You will need to make reservations for a time though, if you do that one. People like to take their time and really read most of it, take it all in. It's incredible. For that reason, they limit how many per hour are allowed entrance. It's still pretty many, but if you don't get a time ahead of time, you will be SOL. They won't let you in because there just aren't any tix left. I loved the natural history and the art museum, but the two I mentioned, there are things in those you can only see in DC. That makes them top of my list if I only were going to take one day to go to the mall. You do have the Kennedy Center not too far away and could do an evening show, if you have time.

The Washington Monument is cool and people bring their kites to fly around it, down on the mound at the base. The Lincoln Memorial is definitely a favorite of mine, but is a bit away from the rest I mentioned. You can stroll the long reflecting pool, made so famous in many movies over the years. If you don't do the Art Museum, at least go down to the food court. The waterfall is really cool.

Also, even if you wanted to drive, there is only one parking space for a million people there. Take the Metro. It pops up all over the mall area and is faster, safe and kinda fun to ride. Traffic and how things are named will drive you nuts if you drive. I tried it not once, but twice. Never again. Also, if any of you are staying outside of DC, on weekends, the park and ride parking fee is free. You just have to pay for the Metro. Well worth it, IMO. I used to be able to ride pretty much wherever, all day, for about $5 each person. I'm sure it's gone up a bit in the last 5 years but still, so much better than driving.