I fecked my knee and had to have surgery, in the 6 months before it I could barely walk, so started swimming. Took about 6 month after before I could ride a bike too.
Like turbo training it can be tediously boring, but if you can find a buddy to go with then it can be quite sociable, even if all you do it follow them up and down and chat for a couple of minutes after each set, that and the friendly competition, a bit like riding in a fast road group.
The other things that worked for me were the cross trainers at the gym for longer sessions and rowing machines (despite rowers suffering with bad knees just as much as cyclists) for shorter (20min) sessions. I found I could often work my away around the gym doing 5min on each cardio machine (elliptical, cross, bike, treadmill, rower) without pain, double up on the least impact ones and I could do an hours solid cardio.
And don’t underestimate walking, it’s still ~400 calories an hour if you can manage a brisk walk.