You know of road biking and mountain biking, but do you know its angry lovechild, cyclocross? A hybrid of the two—check out a cyclocross bike—these races around a short circuit are filled with pavement, sand, mud, barriers, and other obstacles. Bike handling skills, dismounting and running skills are key, as is riding with five pounds of extra mud.
You're going for an hour with all cylinders firing, and sometimes people figure out a way to get a leg up on the competition.
It's currently 'cross season now, so you should probably get out there too.