It is often recommended to put rubber mats or mulch, wood chips, pea gravel, or sand under outdoor play sets. These materials are generally safe enough to fall on and that it can minimize injuries.
Rubber mats and mulch are low maintenance, available in different colors, and strong and durable. However, it can cost higher and may not be suitable for some people because of its off-smell. Sand is cheaper and more natural. However, it can also contain a host of diseases and can cause a mess. Pea gravel may not be as popular as rubber mats, yet it is also durable and natural. However, pea pieces may get stuck in your footwear.