Using the time while brushing your teeth to work out is excellent. In one year you spend about 24h in front of the mirror brushing your teeth. Make sure you spend that time well and make it a habit to do small exercises instead of just starring ar yourself.
Push yourself swiftly up on your toes and slowly back down again. You can increase the resistance by doing it on one foot at the time which will also exercise your balance. This is a great exercise for when brushing your teeth because it takes about two minutes to complete this workout. Aim for about 120-150 reps.