Change something every few minutes. If you break your workout into smaller intervals, instead of just running/walking for one long time at one pace, it will be more interesting, the time will pass faster, and you'll get a better workout.
Run intervals for 15 minutes. Run fast for 1 minute, walk for 1 minute, run fast for 1 minute, walk for 1 minute, etc. etc. The 15 minutes will fly by and you'll burn as many calories, if not more, than if you were running at a steady pace. I've read about several studies that showed that people running intervals lost more fat than people running at a steady pace. I've been doing this two or three days a week for months and it never seems like it takes 15 minutes. I run 8 one-minute sprints in the 15 minutes and I count them down as I go (1 sprint down, seven to go; two sprints down, 6 to go).
Change the incline. Start at 0 and increase it by one every two minutes until you are at the highest incline (at some point you'll have to slow the pace to a walk). Then drop it by one every two minutes until you are back to 0. Incline work is great for your butt.
Do things other than running or walking. My trainer has had me shuffle sideways and bear-crawl on the treadmill. Bear crawling must be really good for you because it is one of his favorite exercises. He has me do it during almost every workout (although not usually on the treadmill). If you try these, start at a very slow pace (1 mph) until you get the hang of it.
Try walking with light hand weights. Do curls, shoulder presses, or lateral raises while walking.
Or try this
treadmill workout from Health.com.