There are so many benefits to doing your cardio work in intervals read my post Why Cardio Intervals are so Dang Effective to understand why!
Now that you are convinced that intervals truly are THE GREATEST, try this amazing interval workout. It can be done on the treadmill, bike, elliptical, row machine, etc. or even outside via running up and down hills or changing the pace of your run! Work it ladies and gents!
5 min walk/jog warm-up
1 min RUN (as fast as you can go!)
1 min jog/walk/or rest
2 min moderate effort (about 60-70% your max)
1 minute jog/walk/or rest
(Repeat 4-5 times)
Be Happy. Be Fit.