Pushups,reclining bicycles, reclining leg lifts or scissor kicks, crunches, hanging or 'uphill' crunches, tricep dips, bridge with leg lifts, plank variations like shoulder-tap or one-leg plank or side plank, leg lifts or hip dips or crunches from side plank, glute kicks from hands and knees... all stuff that's low strain on at least one foot, vigorous enough to feels 'exercise-y' (which is often what people mean by 'cardio', as much as literally raising heart rate), can be adjusted faster/more explosive to feel more vigorous if form is safe, or repeated at length as a cardio endurance workout for someone who's starting in good cardio shape--as long as safe form is is maintained.
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all stuff that's low strain on at least one foot, vigorous enough to feels 'exercise-y' (which is often what people mean by 'cardio', as much as literally raising heart rate), can be adjusted faster/more explosive to feel more vigorous if form is safe, or repeated at length as a cardio endurance workout for someone who's starting in good cardio shape--as long as safe form is is maintained.