Always do intervals! and change the speed every so often to boost your heart rate. Sprinting is also a good method of cardio and works very well. A few exercises that are cardio related besides running are; jump roping, HIIT workouts, swimming or riding a bike. You can be creative with cardio and make it as personable as you want. Not many people like to do cardio but it is a good way to burn fat in short periods of time.