The sand box on the deck proved to be a bad idea. There was always sand in the house and I had to moniter them very closerly to make sure sand wasn't getting in the hair, mouths and eyes. Plus the pool would fill with water and then there would be a wet muddy mess.
We cleaned that all up and brought the kitchen up from the back yard. Then I put the leftover acorns in little containers in the kitchen for the girls to play with. These acorns were from red oaks and didn't peel as easily as the ones from a white oak tree. After painstakingly cracking about 2 cups worth to cook up, I put the rest outside. They are a fun sensory toy and not very messy.