oooh oooh, I can help with this!!
Boxing, punching. For instance. Stand (I'm guessing u can even sit given u have enough room) in a slight squat. Do punches in the air, making sure to twist at the waist for added ab work. Types of punches can be straight punches, undercuts, round punches. If u want to add strength to it. Do same with 3 lb (5 lbs are a bit much for me) hand weights.
Another exercise would be to slightly squat, feet wide again (somewhat horse stance) hold light weights in hands, arms are outstretched to the side and twist as fast as u can. You can alternate slow and fast to last longer, like interval training. It can be done sitting, as well, but needing PLENTY of room for the rotation. I'll have to think about some more.
Can u not bear weight on your feet much at all? I'm thinking of standing still, but kickboxing in the air, i.e. 25 front kicks on each leg, 25 roundhouse kicks each leg, etc. (we do these in our warm up at kung fu, it is definitely cardio). You would need to be able to stand on one foot for stabilization, kicking foot lands after each kick.
Hope that helps.