Using mulch is a very cost effective way to provide undersurfacing with a natural look. Mulch is only suitable for outdoor use and will need topping up from time to time. Make sure you purchase mulch that has been certified for use in playground areas.
Sand is also a very cost effective undersurfacing option which tends to work well in beach locations. Sand may attract cat litter and may also increase wear and tear on painted surfaces due to its abrasive texture.