Parking...
Yes quite a nice place, be prepared to do a lot of walking.
And if you drive there, you might want to work out where you are going to park, some of the places require you to leave your car doors open and the key in the ignition so they can move the cars around while you are gone.
There are no highway signs that tell you which exit to get off on to get to the Smithsonian, and it's bumper to bumper traffic nearly all the time however still worth seeing.