Never having read that book, nor for that matter seen any version of the film, I can't answer directly.
However, from my own experience, nature, in the form of open land with possibilities, gives children an area in which they can play, take risks, have an adventure, learn, make friends and be themselves without adult supervision.
There was an interesting programme on Radio 4 yesterday about adventure playgrounds ... you can listen to it
here and it might give you some ideas.