I realize lots of kids don't get the traditional "back yard" experience. But kids will make what they want out of just about any environment, I believe. Where their imagination plays out shapes them even as they shape their surroundings with the overlay of their inner world.
In the summer, I made my yards prairies and forests; in the winter, tundras and glaciers. More often than not, I was a covered-wagon pioneer. Blazing west, "Oregon or Bust," building [imaginary] shelters from wood and sod. I didn't grow up with siblings my age, so I often created elaborate storylines in which every detail worked exactly as I wanted it to because I was in complete control. Maybe that's why I'm a writer. Or maybe I was already a writer back then.
What were your backyard, or equivalent, worlds like? If you didn't have the traditional back yard, how did your imagination play out?