1) Jump Rope: Yes, you need a little space for this one, but it's really not that hard. It'll get your heart rate up in a hurry, but keep you in a moderate zone. Try doing a speed double jump, jogging, or pretending like you're a skier jumping side to side. You can even work on individual leg strength by jumping on one leg at a time. Maybe decide to do each for 30 seconds to a minute and repeat.
2) Got Stairs? Oh yeah, when I was thinking about what I should do if I was stuck inside, the stairwell was the first to come to mind. It might be a little brutal getting started, but once you find a good pace either climbing or jogging up the stairs, it should be just like using the Stepmill at the health club. And time it...so you can challenge yourself each time you climb!
3) Jumping Jacks, Hula Hooping, or Burpees - A little of these can go a long way, but doing this in a circuit can really mix it up and make your workout fun.
Don't forget that you have to have some motivational music to go along with your workout. Making sure you have good tunes keeps your body moving!