For people with sports injuries, especially knee injuries, it's sometimes important to do low impact excersizes that do not place pressure on the knee or injured joints.

Treadmill running for instance is not recommended (if not ok'ed by your doctor).

Bicycle Riding - there are various forms, whether a static bike in the gym (with different workout levels), spinning bike (for more intense class bound workouts) or real world cycling, which is usually more satisfying, because at least you get some air and scenery.

Fitness Flyer - You've all seen the ads on TV. These machines actually work out the entire body and has little impact on joints. As your local gym instructor for more information. You might look like you're walking snow in the Arctic, but it's a great way of flying off those unwanted pounds.

Stepmaster - Step climbing is a very good excersize for the upper legs and buttocks area. It's high rep cardio workouts and will increase bloodflow to the areas in question. If you want to lose weight around your butt and upper legs, this is the one for you.