You can change the level of difficulty by changing your work-out conditions.

While performing an exercise, gently move your head up and down and side to side while keeping your eyes open. This changing field of vision will place greater demands on your proprioceptive organs. Another, even more difficult variation is to perform the exercises with your eyes closed. Taking away vision, a key input to your balance, will make your other sensors work harder. Make sure you have a spotter!