As long as you hit the Smithsonian then you are good. Make sure to visit the museum of natural history ( i think it is called that) It is my favorite of all of them.
Get a map before hand and most GPS mapping software does not have the DC area mapped properly ,i guess for some weird security reasons.
If you have time i would also drive an hour and go to inner harbour , they have some great seafood and a really nice aquarium ( a Little expensive).
Have fun , especially if you buy a new car.