Toiling away on a treadmill will work up a sweat, but when it comes to allover fitness, sports that engage your entire body will give you the biggest bang for your buck—not to mention the ever-important fun factor. "No machine will work your body the way moving around on a playing field can," says Brad Cardinal, Ph.D., co-director of the sport and exercise lab at Oregon State University. "Any sport that combines all the elements of fitness—cardiovascular fitness, muscular strength, endurance, and flexibility—is going to give you a great workout." Just how…