Have you done a search for Washington, D.C.? I know we've discussed this.
I'm pretty sure Fairfax is on the Metro line so you can go into the district for any of the Smithsonian museums, Arlington National Cemetery, the National Zoo, the Holocaust Museum, etc. Old Town Alexandria has walking tours from the tourist info location; you can walk down to the river; lots of places to eat and shop there.
I've heard the new Newseum is wonderful. If you're into shopping, you can't go wrong with Tyson's Mall or the high end Galleria across the highway. Vienna (nearby) has LV, Hermes, Tiffany and the wonderful Chef Geoff's restaurant.
Someone who lives there can give more specific information. There is no end of things to do in the DC area and Northern Virginia.