We've camped in two parks in the DC metro area. Pohick Bay ( a Norther Virginia Regional Park) was really nice. Deer in our campsite in the morning and evening... even though we were a short drive to the last metro stop on the line. In the park, trails to the waterfront, etc. Nicely sized, fairly private sites, at least in our loop.
We also stayed at Greenbelt National Park in Maryland. Also amazingly quiet and secluded, being a short drive to another metro line (except for the class C above us who ran his generator
all thru dinner and happy hour). I saw my first stink horn fungus here... very odd looking.
I would be happy to camp at either park again.
The proximity to the metro line probably doesn't matter to you. We were in town for two separate Solar
Decathlons, so it was really helpful to us.