Fake Boulders - how to make = boxes covered with garbage bags, then cover the boxes with chicken wire, then cover "rock" shape with about 3 inches of pigmented hypertufa (portland cement, sand, peat moss mix).
A Dry river bed instead of a traditional sand pit. Large and smaller river rock lining a sand river with tree stumps, ferns and toys. This is a home daycare that has a lot of natural play elements in their backyard play environment.
Make smaller rocks from hypertufa by just plopping a half shovel full into plastic shopping bags, twist them closed and lay them on the ground on top of the twist to keep them closed. Two days later, unwrap them, and out they go to your garden bed edge.