I know nobody in a wheel-chair so I'm not sure if this is doable but would they be able to do any hill work? I mean obviously they could just wheel themselves up, and over time try to do it faster? I think that'd be all the cardio exercise they'd need and it would involve "lifting" heavy things too. I would assume the entire upper body would get a pretty good workout and the individual would be spent even if they can't do it so fast. I'm not sure how well it would work but it's a thought.