When short on time you need to use complex movements. These are the ones that burn the most calories. These include squat variations, push, pulls, etc. Things not included would be bicep curls, leg extensions, tricep pressdowns. Not to say these exercises are not effective, but just that they should not be the stapple of your training program. Try this one for size
1. push ups (to failure)
2. plank holds (for 60 seconds)
3. up downs (for 60 seconds)
4. single leg squats (to failure each leg)
5. pullup or pulling variations
Get as many rounds as you can in 20 minutes. This is definetly an intense one and not for the faint at heart. If you do try it, I warned you!