Good question. From a personal viewpoint, the picnic table is the best place. When I am lucky enough to be invited for a BJ, or even a shag I like to be watched. Only trouble is finding the right balance between being accessible, and privacy from casual passers by (and police!). The few places I KNOW would be great aren't used????
Other than that the campers are the best. Room to really enjoy it in 2's, 3's or 4's, and curtains for the more open side, and windows for the watchers.
What do you think?