Although the Natural Playground Company's clients are primarily schools, daycares and communities, we think their philosophy and approach to creating natural playscapes for children can inspire any backyard or outdoor space. They've created over 50 natural playgrounds around the country, primarily in New England, and as you can see from these photos (you can find their portfolio here) they utilize and enhance the natural surroundings - hills, foliage, rocks, water - to design a space that encourages children's desire to play in and interact with nature.
You may not have a hill to build an incredible slide into (photo 3), but simple posts in the ground at different heights are easily achieved and a fun challenge for kids. You get the idea.
You can learn more about the work of the Natural Playgrounds Company as well as explore their large collection of articles and links at their website.
(Images: Natural Playgrounds Company)