Mountain climbers are a great core exercise which can have a bit of intensity thrown in to draw out a bit of a sweat!
The Mountain Climber is done by setting yourself up in a press up position, remembering the straight line we want to see from the top of your head down to your toes and keeping your wrists under your shoulders. Bring one leg at a time in towards your same side elbow and bring it back again. Try to minimise rocking in the shoulders and try to ensure that your leg moves forwards in a straight line.
To increase the intensity you can increase the speed with which you perform the exercise. This will get your heart rate up and make the exercise more physically tiring.