For many children, playing and physical activity are the same thing. Adults, however, often see exercise differently. Many adults assume workouts have to be structured, and have to be "work." But as long as you stay aware of your physical limitations, there's a lot to be said for unstructured, free-form play.
This week when I exercise, I will try "playing" rather than "working out." I will recognize the physical and mental benefits of unstructured, non-goal oriented fun!