An unconventional choice but if you want to unwind there is a Asian-style spa / bathhouse 20mins away in Centreville with pools, hottubs, sauna, fitness center etc, $35 admission for the day.

Spa World

It's in a mini-mall together with Korean restaurants (BBQ, chicken, Korean-Chinese) plus a huge supermarket and an IHOP. You would need a car (or cab ride) to get there from Dulles.

http://g.co/maps/ykyz9