For natural beauty, +1 on Luray Cavens and Shenandoah National Park. There's a nice museum on the settlement of the Valley at Luray Caverns, so that coupled with a leisurely drive through scenic countryside makes for a great daytrip.

Manasas has a cute downtown for wandering, but not much to occupy your time.

There are number of wineries within an hour west of Dulles. Many are by appointment only, so you may want to plan ahead. I wish I could recommend which ones are best to visit... http://www.virginiawine.org/
More sophisticated shopping at Tysons Cornes, Dulles Center, and Fair Oaks.

And of course, the National Air and Space Museum's Udvar-Hazy Center.