I met the couple who run Atlantic Oaks. Great folks. As usual with local attractions, we're a 1/2 hour from there and have yet to stay. I guess we'll need to do something about that.
Sweetwater Forest in Brewster has a few FHU sites. Bourne Scenic Park is usually full all season. People rent out their houses all summer and stay in their fiver there.
September is a great time on the Cape.