First, find a baby that is petrified of the ocean and go for a long walk. Good for the biceps, quads and most of all, your soul.
Then, find an almost four year old who is afraid of heights and climb to the top of a lighthouse.
Add the baby who is not afraid anymore and thinks he can climb up himself and you've burned at least 500 calories.
And if you really want to challenge yourself, wear leather dress shoes that keep slipping off your feet. It will strengthen your toes in a way that you never thought possible.