this is going to sound really stupid ..... but the best thing to do it to build muscle where the damage is to protect it.... (so if you can do some weights light weights ... but 25 reps a set and 3 to 5 sets per exercise, but this is taking it to another level... I could go on forever so back to cardio)
there is not many aerobic exercises that won't aggravate joints in some way....
do you take any joint supplements it may sound like a gimmick but they do work... some of the guys in my gym are 18 to 20 stone and they're joints would take real havoc from just everyday life without them ....
I would say the cycling was about the best to not cause as many problems .... but I would say get one that you can change the resistants....
I would say you want to pedal at a rate that is free on the knee joints without to much resistants .... and change intensity like 5 mins at a comfortable pace..... the 2 mins really pushing it at about 85% do this until you finish your allotted time..... this type of training is very good for cardio.... and not full on either