My favorite Treadmill workout is a 5 minute warmup. Then you run (or walk) and a high intensity for 4 minutes (I usually tell them to kick it up a notch the last minute), then quickly jump off the tread and do 20 squats with weights -- drop weight to tops of thighs on the way down and push weight over head on the way up. Then do 10 push-ups at the end of the treadmill. Repeat for 6 sets. By the end, particpants are dying! Cool down with some shuffles and skips and then stretch.
Please send your tread workouts, I would love to share!