If you can't maintain even a moderate level of intensity while exercising due to boredom, you will probably run into problems if your goal is weight loss or overall improved fitness levels.
For cardio workouts, you generally get the most benefits when your session extends past 20 minutes.
In general, sustained cardio workouts are the best way to boost overall metabolism so that you're burning more energy, even when at rest, but even moderate weight lifting programs designed to build muscle mass and tone will do the same. Muscle takes energy to sustain; that's why competitive weight lifters consume so many calories. They would start losing weight if they didn't.
Treadmill workouts are among the most boring workouts though; no question there. Unless you can lose yourself in your workout music, or you can watch your favorite program or movie while running in place, it can be pretty dull. But, it all comes down to your personal goals and how much you're willing to work at them. Generally, there are no simple solutions or short cuts without effort or everyone in the world who didn't specifically wish to be out of shape would look like fitness trainers or professional athletes.
If boredom is your real problem rather than lack of motivation; mix it up. You don't have to do all your cardio workouts on a treadmill. And if you live in cold weather conditions, you will burn more energy working out in the cold. You'll have to burn up more energy to keep warm.
