[Closed] Seated cardiovascular exercises

So there are a number of us with lower limb injuries it seems. I'm waiting for surgery on my knee and I'm going to be off the mountain bike for a year (and it's already been 5 months since the orginial injury!). I'm putting on weight and going mad at the lack of exercise.

Any suggestions for CV workouts without putting any load through my knee? I'm not allowed to swim at the moment (probably not for a few months).

I don't really want to pay a gym membership each month just to use the hand cycle for 10 minutes a couple of times a week!