The knees (and other joints) can benefit by load-bearing and strength exercises, of couse. (So long as it's not injurious levels of exercise on those areas.) That said ...

I've found hand-oriented cardio like a "hand cycle" can work (ie, SciFit's variation). If the intensity's sufficient for a cardio response. There are gym type cycle equipment that also has arms, which allows a basic seated cycling position with cranks and arm levers. Rowing is also a low-impact, low-strength way to boost cardio while keeping a minimum load on the knees. It requiures movement of the knees, yes, but the seating position and motion (on rails) keeps pressures to a minimum. And there's swimming. Any of these can be done in such a manner as to induce a range of cardio challenge.

If winging it, not in a gym with such equipment, there's always brisk "ruck" marches along local trails. If you've got hills in your area, and if you can walk ~5mi of distance, a weighted day pack or ruck sack and brisk speed can yield surprising cardio benefit. Wouldn't be any more injurious than walking, assuming no injuries (or even inflamed/weakened joints) to cope with.


There is also a exercise "circuit" approach. Focusing on "floor" exercises, it's surprising how a combination (if done briskly enough) can yield a solid cardio impact. And most of these won't be impactful on bum knee, elbow, shoulder joints (depending on the exercises selected). Some example exercises and ideas for "circuit" combinations of them:

https://darebee.com/workout/types/cardio.html