What kind of cardio can I do with gimped up knees?

Keep in mind that it is really difficult for me to get to a pool which I know would probably be the best thing.

I bruised my kneecaps (the insides of them) two months ago and I was told by my doctor that it could take up to 6 months to heal. He advised that I bike but I have found that even bike riding makes them hurt.

I have just recovered from fasciotomy surgery on my anterior tibialas muscles that will eventually allow me to start running again pain free. I really want to try running but I really don’t want to re-damage my knee caps because it takes so damn long for them to repair.

Any cardio exercises that you can suggest? I am staring down a pool for my only cardio for the next few months?