You all know I am a fan of cross training since it adds a nice variety to a workout routine. Even if you don't cross train, you should vary your workouts to reap even more benefits from your cardio time.
You need to play with speed, distance, and rate and mix it all up. This variety confuses your body and helps you burn more calories as well as increase your fitness level.
So mix up short and intense runs, less distance at a faster pace, with longer and slower runs to work on endurance – this variation works for cardio machines, too. If you run outside vary your terrain, doing some hill runs mixed with runs with less inclines. This also applies to the treadmill; you need to play around with the incline. And don't forget about intervals. Mix up short and long intervals of pushing your pace and then slowing down to recover.
With this much variety in your workout plan, you will never get bored and neither will your body!