If you want a unique fitness challenge that will get you in even greater shape, consider boot camp workouts. Boot camp classes are modeled on the basic training programs used by the military, with push-ups, sit-ups, running, jumping and climbing. Sometimes these classes are held outdoors, but a number of gyms offer indoor boot camp workouts, which can be more convenient.
Similarly, there are boot camp classes that mimic fire fighters' training; you may yourself find lifting and dragging weighted bags to simulate hauling heavy hoses or rescuing people from burning buildings. There are yoga boot camps and ballet boot camps, too, that combine yoga or ballet moves with more aerobic activity and extra abdominal work. Some boot camp workouts focus very specifically on a technique, such as the subset of yoga poses that involve arm balance.
Boot camp workouts are not for beginners--these are tough classes, and even experienced exercisers are likely to feel soreness in muscles they didn't even know they had. That said, these group workouts will get you in great shape thanks to major calorie burning.