free tires from garages more then willing to give them away. A few cans of spray paint from Walmart (paint after assembled to save on paint) and Cypress mulch (packs really nicely) from Home Depot. Adding in a Little Tikes sliding board.
Interesting idea for putting a play area on asphalt, concrete or hard soil in a temporary build. The tires and rocks can be removed, and the sand and rocks swept up. Interesting idea for day cares in urban settings without access to grass or soil.