Deadmill sprints are #1 indoor HIIT (high intensity interval training) exercise. All you need is a treadmill and some drive!
Keep the treadmill on but leave the speed at zero. Set the incline anywhere between 9-12. Hold onto the handles in front and drive the belt with your legs for 10-15 seconds. Rest for 40-60 seconds and repeat for 10-15 total rounds.
Editors note: I had to pause my treadmill to free up the belt to push it. It may be different on each treadmill. Find out which works best for you.