I helped with the design of a 7 acre park in our neighborhood, here were some of my ideas:
1. Make a "play structure" from logs and boulders laying on the ground. As the logs rot, the kids & creatures will pick them apart looking for insects & satisfying their curiosity about the decomposition process. When the logs have totally decomposed, bring in new ones.
2. Make a living play structure by partially cutting down a tree. If there is an existing large tree that is leaning, cut the uphill
roots and pull the tree over. This will provide a mostly horizontal climbing surface that will continue to grow (assuming the downhill roots do not break off when you pull the tree over).
3. Create a slow-dying snag for wildlife. See "Creating Snags from Live Trees" in this document:
http://wdfw.wa.gov/living/snags/snags.pdf
4. Use an “ecology
lawn mix” instead of regular grass.
5. Capture storm
water runoff to use for irrigation.
6. Consider using a “living roof” for roofed structures.
7. Rope swing(s) from tree(s). Most kids these days grow up without ever seeing a rope swing.
8. Position benches and other features to encourage people (parents/teachers) to talk to each other. So often benches are not near each other, so people don't get to know each other.
9. Plant a big patch of sunflowers, and let them go to seed so the kids can watch the birds eat the seeds. Buy black oil sunflower bird seed and plant it - way cheaper than buying it as "garden seed".
10. Plant wildlife-friendly
native plants. Provide wildlife trails across the site.
11. Places to play in dirt and/or water.
This was before I had heard of
permaculture, otherwise I would have suggested edible plants too. I have most of these features in my yard. The horizontal tree (a big old walnut that fell over during an ice storm - the uphill
root broke) is the coolest play structure in the neighborhood. Later we ended up putting a tree house in it.
The design committee ended up doing 1, 4, 10, and a few of my other ideas that I did not list. The rock/log play "structure" morphed into a "discovery garden" that is probably the most popular feature of the park. They made a sort of zig-zag/cluster pattern of logs and boulders interspersed with plants. The kids love climbing around on it. Here are some photos taken during the winter when the plants are all dormant:
(see
http://s831.photobucket.com/albums/zz238/behindthewaterfall/Discovery%20Garden/?albumview=slideshow for all photos). I see toddlers playing here while mom & dad sit on the rocks & watch; teenagers horsing around jumping from rock to log to rock; elementary kids using sticks & branches to build a "fort"; even a family getting their portrait taken among the boulders.
A climbing tree is probably too much for a preschool, but perhaps you can plan ahead the kids' elementary school, park, or kid-friendly backyard?
If the site does not currently have any trees, plant some - to use as snags, logs for the discovery garden, as a source of leaf mulch/food, a future horizontal play structure, etc.