Sounds like you're not planning for a car, but just in case, you might be interested in the national zoo (for the famous pandas) or the other part of the air and space museum (near Dulles - it's a hangar full of cool planes).
I actually am driving down there but won't be using the car unless necessary. Question is how much I can fit into my time frame
Speaking of cars in DC, anyone with any good parking tips?
TIA