A place designed for play should leave room for play its designer did not imagine.

Play is invention, and a space that specifies every use has already done the inventing. Children treat a wall as a goal, a slope as a race, and a bench as a stage, and none of those were on the plan. Over-designed spaces are often perfectly safe and slightly disappointing, because there is nothing left to make up.

Wherever you have some say over how a space is used, leave part of it undefined. The same applies to time with children, where an unscheduled hour usually produces more than a planned activity.