Here's a plug for an inspiring blog about plaground design: Playscapes and an introduction to "natural playgrounds." From Playscapes:
"In general, natural playground spaces tend to be custom solutions that emphasize raw materials of wood, stone, and sand in combination with shaped earth forms. They often include elements of water play, and have more emphasis on planted material than a typical playground. They may feature elements for learning about the natural world, like bat houses or bug habitats, and the boundary between ‘playground’ and ‘garden’ or ‘nature trail’ is blurred. It should be noted that some landscape architects have always worked in these elements, without designating themselves specifically as ‘natural playground’ providers."
I look at it this way: natural playgrounds are the boxes that the expensive Christmas presents come in, you know, the boxes that the kids love playing with more than the toys? My kids spend hours in their sandbox, full of river rocks, pieces of wood for building, and surrounded by cool plants. They love the sassafras trunk built into their traditional wooden swing set. Mud and water, the hose on a fine mist setting, they're in heaven. Happy squealing kiddos.
The academics can give all the reasons why natural playgrounds foster creativity and healthy child development, but the happy squeals are a pretty good indicator of when it's done right.