You know, if you watch any wildlife shows on TV, this kind of free-form playing/exploring/imagining is an IMPORTANT part of growing up for any higher species. I really worry about today's kids who can't play a game unless it is on a computer or some adults are telling them how to play. Always have to follow the rules, never get to make up their own rules. Five-year-olds in organized team sports. They are not learning any life skills.
We used to play kickball, we picked who would be the captains, they picked sides, we refereed our own games. No adults around. I was one of the ones usually picked last, but guess what? I survived with my self-esteem intact.