I agree that taking the car into the city after rush hour isn't a bad idea, but the roads can be a bit confusing getting back out of town if you're not used to them.

If you decide to stick with Metro, I might suggest a circular trip with an arrival at Smithsonian Metro stop and then a departure from Foggy Bottom back to Crystal City. It will take a few extra minutes to get to Smithsonian, but the view as you leave the station in the middle of the Mall is fantastic, with the Capitol in one direction and the Washington Monument in the other.

Then you can walk down towards the Washington Monument and get a close-up view of same, walk through the WWII memorial, and visit the Vietnam and Korean Memorials as you reach the area of the Lincoln Memorial.

Then it's a fairly short walk (Maybe 5-6 blocks) to the Foggy Bottom station for the return "home".