We visited these places on a gray, rainy day which was kind of nice. Pohick Church was both George Washington's and George Mason's church and it was a good spot to visit, both for colonial and Civil War history. The grounds are lovely too and it's a pleasant spot.
Gunston Hall, George Mason's home, is a few miles away. The grounds are fairly large and there's a small visitor centers, but the main thing to do there is tour the house. We were pretty much the only people around that day so they did a tour just for the two of us and it was really interesting.
The area was lovely too, quiet and rural, despite being so close to DC.