Invite a trainer to your house. DVR a workout show and fast-forward through the commercials. Most half-hour workout shows are really only about 20 minutes long without the commercial breaks, and they include a warm-up and cool down. It's enough to get your heart pumping without taking too much time out of your day. Best of all, you don't even have to leave the house to do this one.
Power clean. Who says cleaning isn't a workout? Not someone who's had a half hour to get their house spic-and-span before guests arrive. Pushing a vacuum or hauling one up stairs is a workout in and of itself. Polishing windows and tables takes some elbow grease. If you need to get some cleaning done and you're pressed for time, try setting the microwave timer for 20 minutes and moving as fast as you can. The upside is you'll also get more cleaning done in a shorter amount of time!