There are a bunch of restaurants and shops within walking distance at Reston Town Center. Feel free to go for an afternoon stroll after lunch, get some shopping done or catch a movie!
DC is about a 40 minute drive from Reston. If you plan on staying in town a bit longer on Sunday, check out the Smithsonian Museums (which are free!) or any of the monuments. There is also a metro stop in Reston which will take you into the city (Silver line).
If you enjoy nature, you may be able to squeeze in a quick hike this weekend at Great Falls National Park. This is about a 20 minute drive from Reston.