When I don't have much time for a workout, I'll do a quick circuit to get my heart rate up. This is a typical one I might do, you can adjust the number of each move based on your fitness level. If you have time, you can repeat this circuit as many times as possible with a minute or two of rest in between each.
No excuses, this circuit doesn't take long at all, doesn't require any equipment, can be done anywhere and is modifiable based on your body and your time available. With no rest in between moves, this circuit will take you just under 3 minutes! Rest for 2 minutes in between circuits and complete 4 times for an awesome 20 minute workout.