There is not much public transit in that area, and none that would take you up to DC in the morning and back in the evening. If you don't want to spend too many hours in DC and are visiting on a weekday you could drive to either Manassas or Fredricksburg and take Virginia Railway Express into the city. This is a commuter rail line that runs only on weekdays and that has a somewhat limited timetable! so check times before you plan to use that option.
On a weekend, or if you want to travel at hours VRE doesn't run, I would either drive into the city and pay to park, or drive to a Metro station like Franconia/Springfield or Vienna and take Metro in. The Metro option isn't great on weekdays because many of the Metro parking lots fill. If you do plan to take Metro on a weekend, check www.wmata.com for updates on any track work that weekend. Some stations / line segments may be closed for construction.