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.