A good one is water work outs. like: water aerobics or swimming it's great for weight lost and it reduces impact on the joints
Warmups are important for any routine, maybe even more so if you have bad knees. A proper warmup helps to loosen stiff muscles and get your blood flowing. To get your body ready for the exercises that lie ahead, do some brisk walking or other light activity that doesn't hurt your knees until you break a sweat. Then, perform some static stretches to loosen the muscles and ligaments before you begin. Take 10 to 15 minutes to thoroughly warm up your body, or follow a specific routine as set down by your physical therapist, if you have one.
Swimming is gentle on all your joints. You can perform most of the swimming strokes to swim laps of the pool, tread water or just move around as you please to exercise without impact on your knees. Avoid any strokes that cause discomfort, and swim continuously for at least 20 minutes so your heart rate is elevated and the fat burning effects of the exercise take hold.
Riding a bike is another non-impact exercise that is beneficial to weight loss. Riding a bike utilizes your quadriceps, hamstrings, calves and glutes to propel you forward without any pounding on your joints. To get a quality weight loss workout, ride your bike off road or on the road, or use a stationary bike at the gym. Stay away from steep hills to protect your knees; riding on flatter surfaces doesn't require as much intensity.
Despite some people's belief that weight training is for building big muscles, it's also a beneficial weight loss exercise that someone with bad knees can use. Leg exercises like heavy squats, leg presses and leg extensions will likely have to be avoided, but you can still get a good calorie burn with others. The bench press, shoulder press, rowing movements, biceps and triceps exercises and abdominal work helps to build lean muscle, which helps burn fat. Set up a series of exercises in a circuit to increase the calorie burn even more. Use caution with leg exercises and lift your feet off the floor if you turn while holding a weight so you don't twist your knee.
I've found that yoga is wonderful for my knees. It stretches out my legs, while strengthening them so that my knees do not have to work as hard. Everything in your body is connected: for example, when you work on stretching and strengthening your hips you are helping your back and legs.
I also use braces and athletic tape. I wrap athletic tape around my bad knee and an ankle brace on my bad foot. I've gotten to the point where I can run 3-8 miles a week. I would also recommend wearing good shoes and replacing them regularly.