'Step-ups' are gentle exercises once you are mobile - to start building up quads strength and a bit of balance. 



In case you cannot see the video, here is a still sequence -


Stand in a relaxed position in front of a low step.

step ups

Step up onto the step with your bad leg, and then step up with the good leg...

step ups

...to stand with both legs on the step.

step ups

Step down again starting with the bad leg, and follow with the good leg to stand back on the ground again.

Start the regime with 10 reps. If it is too easy you can use higher steps or small ankle weights.