If you go to Leesburg, stop by the visitor center and pick up a walking map of Waterford, which is an interesting historic area.

The museum in Old Town Manassas offers up a free walking tour brochure, too.

A bit further afield, but still just about an hour's drive from Dulles, Harper's Ferry is a nice day trip. Or Frederick MD has a ton of historic sites.

Depending on the time of year, you might also consider hiking in the Shenandoah area.

Or what about Luray Caverns? Or gambling at Charles Town Races and Slots?