If you're looking for a way to make your cardio program more intense, consider interval training. Most cardio machines have a "random" or "interval" setting. Next time you're doing cardio at the gym try one of these programs out.
If you find that a given program is not the right intensity consider creating your own interval program. If you are on the treadmill, for instance, try running at a fast pace for 30-45 seconds followed by 60-90 seconds of a fast paced walk. Increase to a 1:1 ratio if you're looking for higher intensity. A 2:1 ratio makes for an even tougher workout!
Interval training is beneficial in keeping your body challenged during a workout. The uncertainty of change, whether it be in speed, incline, or resistance, can take your standard cardio exercise to a whole new level.
Try implementing an interval program on the treadmill, stationary bike, stairmaster, elliptical, or rowing machine next time you're in the gym for cardio. Depending on how hard you push yourself it can be a great way to ease into a new type of cardio or bring your typical cardio exercise to a much higher intensity.