Keeping a bunch of kids occupied in summer

Slip n Slide, water channels with aqua ducts using PVC for boat race, etc.

Make a cubby, build a pond, build a dry creek, a fairy garden (or miniature garden), build a mud kitchen, build an underground green house, etc.

Build a willow playhouse but for squash or some other vegetable creeper so they can play in it and have vegetables to harvest at the same time.