I have basically zero time for working out right now as I try to navigate through Steam and get GS launched. So for the last week or so, I've been doing a few sets of a workout I'd seen months ago, saved, but never tried. It's a 7-minute circuit training workout, with 12 30-second stations and 10 seconds in-between.
The basic principle is that you don't give your body quite enough time to recover, so if you do two sets (or three), you wind up getting a decently intense workout on a day when you otherwise wouldn't have worked out at all.
Here's a link, with a nice graphic that you can print out: 7 Minute Workout.