Natural play areas often look like miniature natural landscapes.
Flat land is transformed into rolling hills, new moss-covered stone walls look like they've been there forever, vegetation grows where it looks right, weathered brick paths meander around "natural" features, sand play areas are scooped into the ground, fallen, or even "upside down" trees are perfect for climbing, natural rock piles make wonderful seats and climbing areas, loose parts for constructing things are all over the place...
and from Wikipedia:
“Natural Playgrounds are purposely complex interplays of land shapes, structures, and vegetation that fit with the land in ways that challenge and fascinate children and teach them about the wonders and intricacies of the natural world while they play within it.” (written for Wikipedia by Ron King in 2009)
Please review How are Natural Playgrounds different? for more information.