You don't give us times to go by. You want to leave downtown by what time? What time is your flight from Dulles? Need extra time to return the car and take the shuttle to the terminal, of course. Exact times make a big difference. And how much driving are you willing to do? Harper's Ferry might indeed be doable with an early start, and a late night flight. Leesburg probably easier. Frederick doable. Luray a bit of a stretch, I think. You might look up directions to take the toll road Dulles Greenway (?) from the north to get into Dulles, avoiding the traffic on the Beltway leaving the DC area in the late afternoon/evening. There's also the National Air and Space Museum's Steven Udvar-Hazy Center, very close to Dulles (parking free after 4, I believe).