Check out anything by Robin C. Moore & Herb H. Wong - specifically "Natural Learning- Creating Environments for Rediscovering Nature's Way of Teaching." Another excellent resource and perhaps more specific to what you are looking for is "Play for All Guidelines - Planning, Design and Management of Outdoor Play Settings for Children." I have the second edition of this book.
There are so many simple, creative things you CAN do that actually end up being more cost effective and inspire creative play and connection to the natural world. These books are reference books with hundreds of ideas.
Hi Laurie and thanks for your question. There is a new early learning guide from Project WILD called Growing Up Wild which has been blessed by NAEYC. Here is the web site for more information on how to get the guide and some training: http://www.projectwild.org/GrowingUpWILD.htm
Also, there is a wonderful booklet from the National Arbor Day Foundation called Learning With Nature Idea Book that describes natural play areas for young children. Their web site is www.arborday.org and you can contact Susie Wirth there for more information at email@example.com.
My thanks to Lulu and Diane for their suggestions as well. John
It isn't clear whether the tripping hazards are an overzealous interpretation of health and safety laws or whether the ideas you are thinking about have inherent big "trip and slip" factors. Perhaps if you share the ideas you have got and we may have suggestions about how these ideas can be done within H&S parameters. Here in the European Union, all the countries have the same H&S legislation but the interpretation between and within countries is quite remarkable.
Also useful thing to bear in mind is the routines within your child care centre. This can minimise the hazards posed. Another thing to consider is the benefits of the ideas that you have...in other words if the benefits from the activity really outweigh the risks then there maybe ways to reduce the trip factors.
Finally is there an option of changing the use of the area. Attached is a photo of a play area where decking was put in around the trees as the school principal was worried about children tripping on the tree roots. The additional space is big enough for 30 children to sit on (at 2 levels), lots to lie on and even an outdoor performance area! It was officially opened with a "Chillout Rap"