Hi Riley. First and foremost, have your knee pain checked out by your doctor to see what the actual problem is and whether you need to stay away from ALL cardio exercise involving your lower body. If you’re cleared to at least do low-impact (or no-impact) lower body cardio, then the bike is a good option (set on an easy setting). Otherwise, swimming and as Karin mentioned, the upper body ergometer (one of my personal favorites) are both good options.
I hope that this helps.
1. Find out what the cause of the knee pain is.
2. Get it resolved.
3. Go back to doing the cardio you enjoy.
4. Make sure you avoid what caused the knee pain.
On another note…
I personally enjoy Kangoo Jumps, the take the stress off my knees.
It really depends on the problem. Knee pain is very vague. Let me help you narrow it down. Where on the knee is it hurting(front, side, back)? What kind of pain is it (dull, sharp, burning)? Is it a chronic pain or an acute pain? What movements cause you the most pain?
Depending on how you answer any one of these questions will lead to a different type of training, but in general non load bearing cardio would probably be best.
I suggest you have a thorough functional/mobility assessment done for your hip and ankle. Most problems that are not caused by direct trauma are related to issues/compensation patterns in the kinetic chain.
Knee pain, as multiple people have mentioned, could have a variety of sources. I would suggest visiting your doctor and seeing if he or she thinks that Physical Therapy might be a good idea. From personal experience, PT did not work for me. I have found that running is not tolerable, on a long-term basis. On the other hand I have found upright or recumbent cycling, incline treadmill walking, hiking, or elliptical machines to be very tolerable.
Unfortunately, we don’t have it all figured when it comes to knee pain. While Physical Therapy can be great and may work well for you, I’ve also seen it fail to decrease pain in others. I hope that you find something that works for you!