Should try an Atlantic Grand Prix event. Kind of a cross between old natural terrain Motocross / Scrambles and modern Hare Scrambles.
Check out the reviews at
www.quadzone.com or even the race coverage at
www.cyclenews.com in regional events.
www.thumpertalk.com has a thread with lots of pictures as well. Even a helmut cam of one lap.
they have their own website:
www.atlanticgrandprix.com
Bikes Saturday, Quads Sunday. You MX'ers can race Saturday and then do your own series Sunday if you have the energy!