The three options below all require little or no equipment, can be done just about anywhere, and are all under 20 minutes.
If you have 4 minutes – the Tabata
Put together by a Japanese professor, a Tabata is a 20-second burst of very, very high intensity, big muscle exercise, followed by a rest period of 10 seconds. The whole sequence is repeated 8 times for a total of four minutes. Yes, four minutes. It works, but only if you truly go high intensity for the 20 seconds, and involve large muscle groups. Good exercises for Tabata intervals are squats, jumping jacks, jumping rope, or my personal painful favourite, the burpee.
If you have 8 minutes – the 8 minute workout
This series of 8 Minute Workouts focus on a single area for 8 minutes. There’s a workout for legs, thighs and hips, abs, arms, and a stretching workout. You can do one each day, and you will begin to see results in a few weeks. I’ve used them all five of the workouts, and even keep them on my phone in case the mood strikes …
If you have 20 minutes – Jillian’s shred
Called the 30-day shred and conceived by Jillian Michaels, trainer to the contestants on The Biggest Loser, this 20-minute workout comes in three levels. I dare you to stick with it for 30 days, doing Level 1 for 10 days, Level 2 for the next 10 days, and finishing up with Level 3. It will kick your behind, even in Level 1. Trust me on this. I had incredulous sweat and sore-in-a-good-way feeling to prove it. Yes, even in Level 1.
These are all short enough to do every day, and, all other things being equal (like your diet), you should start to see small but definite results within 10 days to 2 weeks.