What is the cause of your knee pain? That answer can direct people to help you better.
If it's a cartilege break down, Osteo Biflex can help.
If it's tendon problems, there's little that can be done outside of surgery, which I'm sure you know.
I suppose if the answer lies in doing nothing that involves the knees, you're in a tough situation unless you have a pool. Swimming would be your best answer for cardio.
So let's say there is no pool available. Are you able to do low/no impact stationary cardio? Rapid knee raises or butt kicks are a way to get the heart rate up with no knee impact.
Weight training should be relatively easy to do with bad knees. Push ups, bench presses, flies (peck and rear delt) rows (regular and upright and row to tri-extension), everything shoulder (front and side lat raises, praise raise, shoulder press), many types of bicep curls (regular, hammer, super slows, palms down), and don't forget the umpteem ways to work your core.
If you lift opposite body parts with no rest in between sets, you will get the same cardio benefit as running.