Suburban Virginia
Few visitors to Washington, D.C. will venture out into Northern Virginia, but you should consider it for one specific reason: one of the best aviation museums in the world is out there. The Smithsonian Air and Space Museum Udvar-Hazy Center is a massive hangar that holds air and spacecraft that just plain wouldn't fit in the central D.C. museum.
Whether it's to see the Enola Gay, SR-71 Blackbird, or the Space Shuttle Discovery, it is well worth the drive.