Having lots of playground equipment seems desirable, but this article suggest investing in some natural areas.

Recess in an outdoor natural environment is more restorative than recess in an outdoor built environment. Cognitive assessment results demonstrated better sustained and selective attention and children reported greater restorativeness in an outdoor natural environment than in an indoor built environment. This finding applied to both group playtime and individual free play." | Amicone et al. 2018. Green breaks: The restorative effect of the school environment's green areas on children's cognitive performance.