(3:49 in the above video)
The Med Ball Push-Up puts your hands in a much narrower position while adding a stability component at the same time.
Not only will your triceps take the brunt of the work, but you'll also have to squeeze your chest and your core to remain stable on the ball.
I recommend starting with the feet in a wider base to make sure you stay balanced throughout the movement.
An easy to way to quickly intensify this exercise is to really slow down both the concentric (pushing) and eccentric (lowering) portions of the exercise.