You may think you're too busy to work out, but chances are, you're just making excuses for yourself. More than likely, you have have time to work out. If something is enough of a priority to you, you will make time for it. If you can sincerely tell me that you don't have at least an hour or two in the course of the day of free time, then maybe you're really too busy for exercise. However, for 99.9% of the population, that is not the case.
This post is not intended to make you feel guilty--but rather to help you realize how easy it is to get a short, efficient workout right in your own home. You don't need to spend 3 hours in the gym to get a great workout. If you have half an hour and some floor space to spare, you'll be able to kick your own butt into shape.
You can use circuits or intervals to get the most out of a short workout. If you don't have access to weights, use your own body. Exercises like burpees, push-ups, v-ups, air squats, lunges, and plyometric (or explosive) movements can provide you with significant results. One great interval workout I like to do is called Tabata. Tabata intervals are 20 seconds of work, followed by 10 seconds of rest for 8 rounds. For example: start by doing as many push-ups as you can in 20 seconds, then rest for 10 seconds. Repeat this 8 times. You can do this type of interval with any exercise, and you can even play around with the amount of time in the intervals. You may want to try doing 1 minute of work followed by 20 seconds of rest. Find out what works for you. Tabata intervals are 4 minutes long. If you do four separate Tabata intervals, that means you've gotten a great workout done in only 16 minutes. Now if you don't have 16 minutes of free time throughout the day, you're either lying to yourself, or you're far too busy and you need to let yourself relax a little bit!
I'll end with a great quote from Florence Nightingale:
"I attribute my success to this: I never gave or took any excuse."