I like the idea of pohick bay, it is definitely more my style, and despite being further south, it is about the same distance time-wise. 20 min drive to either mt. vernon or the nearest rail station into dc.
As for the timeframe, we haven't nailed it down yet. The 4th was just an idea because of holiday days at work and the kids are out of school. Plus it would be neat to be in D.C. for the 4th. But not set in stone yet.
Vermont and NH are something we want to do at some point, but I think we want to do that sans children, at least at first. It is a good idea though. :)