Ok, so here is the short and to the point version.
As a kid I had great fun with mostly just my imagination.  I lived in my own little world. ;)  Most often, I was an explorer, a spy, or a martial artist/soldier. The broken riding lawnmower in the back yard was at times a spaceship, a speedboat, or even a tank.  I just used my imagination!  Another example, the dirt hills by the pond.  They could be snowy mountains, sand dunes, the amazon, or even another planet.  I had the great fortune of having playgrounds and parks close by as well.  I had tons of fun whether with friends or just by myself.  So on to the game idea...
(None of this is canon.  Just meant to show an idea)
The general idea: In real life there is a park, but everything is affected by your imagination!  As you choose which character to be, the scenery and real life objects around you conform.  To finish the game you will have to explore the park in each new imaginary world.