There are a lot of great workouts that you can accomplish within a 20 min. period of time. For example, let's take the following exercises… Jump rope, plank, squats, push-ups, situps.
What you can then do is create a circuit… 60 seconds jump rope, 60 seconds plank, 15 repetitions of squats, 15 repetitions push-ups, 15 repetitions sit ups. This would be one circuit; then, complete as many circuits as you can in your 20 min. Circuits like this are designed to be fast paced, therefore, there is no rest in between each exercise. Try this out, I think you will find this to be a great workout.
You can also try this workout… Jog for 90 seconds, then, perform walking lunges for 30 seconds; continue to alternate the 90 seconds jog and the 30 seconds walking lunges for the 20 min. Your legs will be screaming by the end of this one!
Another great option is a workout that I design called 5-Round Fury Fitness. I designed this for individuals like yourself who are looking to accomplish a high quality workout in a short period of time. 5-Round Fury Fitness will provide you with a new and random workout each time, and, there is no equipment needed. Here is the link,https://itunes.apple.com/us/app/5-round-fury-fitness/id512222910?ls=1&mt=8