My knees are super screwed up (I can't even bike) and I'm working on losing weight, too. Got one of these fellas that I try to spend at least 15 minutes at least 5 mornings a week beating the crap out of. His name's Bob. "Body Opponent Bag".
He's got 200 lb of playground sand in his base and I wear a pair of these when I'm working him over:
So far I've lost about 15 pounds. It's the only cardio workout I can come up with that I can do year 'round. When it gets cooler, I'll supplement the workouts with splitting firewood.