What about some balloons that the older kids can fill themselves with water and have a water bomb fight? Letting them fill the balloons themselves will help entertain them a bit lol.

A bit of plastic on the ground for a water slide?

What about something like twister?

Maybe bake some cupcakes, provide some lollies for them to decorate their own mini cakes (DD's class did this at school last term, and named it The Masterchef cupcake cook off"

DVD's, music, playstation/wii etc??