M.R.
My vote would be for the treadmill, along with an exercise/stability ball and some resistance tubing/bands.
The treadmill is so versatile; you can walk, you can run, you can change the incline to focus on the rear and make the workout harder. Park that treadmill right in front of the tv, watch a show, and before you know it 30-60 minutes have passed!
You could also go to www.sparkpeople.com to learn about the tons of exercises that can be done in the house without any exercise equipment at all. Another good book is Fitness for Dummies and Weight Training for Dummies - not that you're a dummy; it's just presented in a super-easy, straightforward manner.