One of the fundamental principles of exercise is that in order for your body to get the best response to a workout, it needs to be shocked. Doing the same routine over and over again, even if at a higher intensity, eventually leads to a plateauing of results as your body becomes acclimated.
To improve overall results and break out of stagnation, try mixing up your routine. If you are a regular at spin classes but haven't been on an elliptical machine for years you might find a mere 15 minutes on the new machine challenging, despite being in good cardiovascular shape. The difficulty of the new movement will payoff later as your body responds to this unexpected activity.
Another variation is to do multiple kinds of cardio in one session. Instead of just doing the treadmill for 30 minutes, try 10 minutes of jogging, followed by 10 minutes of biking followed by 10 minutes of rowing. Even if the machines' readout of the caloric burn from the different approaches rounds out to be the same you will get a much more intense workout.
If for practical reasons you are confined to just a single activity, you can still prevent your body from getting too comfortable by constantly changing your workout profile. If you are using an exercise machine, try an interval training program. This will drive your heart-rate up and down throughout the workout and as countless studies have shown over the course of decades, lead to a higher caloric burn and a better body response.