Newspaper Play Structures -- kids can build & re-build them. Throw a sheet overtop for an instant reading nook. :-)
Also on these boards
Preschool game day activities - keeping little ones active outdoors!
let the children play: taking music play outside: part 1
Use straws to build a marble run. Great for building thinking skills - and yet it feels like just play!
Simple Activities for Little Engineers | via Inner Child Fun