Okay - so you don't have a lot of time to exercise and you feel like you can't get a good enough workout in, in say - 20-30 minutes? Try this...
10 push ups (on your knees if you need to)
10 dumbbell bent over rows - use a band if your don't have dumbbells
10 walking lunges - if lunges bother you, do body weight squats
10 sit ups
Then do some sort of cardio... either run around the block - do 1/4 of a mile - or jump rope (150 jumps), if you have an elliptical at home or a rower - do those for a specific distance
Repeat this circuit as MANY times as you can in 20 minutes. Set the timer and GO! The timed aspect of this workout is what will drive the intensity up.
Keep track of how many circuits or rounds you were able to do in that time, so the next time you do the workout, you are going to try to beat that number.
Now of course - as with any workout regimen - if you need clearance from your Doctor - please do so. Keep it safe - the goal is to keep you around for a while!!