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.
If you have a gym membership, try different cardio machines to get an idea of which machine(s) may be tolerable. Assuming you are treating the injured knee based on a medical diagnosis, then this Gould only be a temporary issue and you’ll soon have the mobility you once had to enjoy a wider range of cardio options.
I saw one of your earlier questions was about Tough Mudder training. Do you still plan to do a race? Depending on your injury you will need to rehab your knee first and avoid any running for now (especially on uneven surfaces). Swimming is a great alternative and perhaps running/walking in the water will help with your cardio. Like Karin suggested, upper body ergometer could be an excellent alternative as well. Have you done any PT yet? Physical therapists can give you some great tips in getting your knee back in shape and work on any imbalances you might have. Muscle activation therapy can also be beneficial for you.
I hope this helps.