Preschool summer camp introduced us to moon sand. If you're not familiar, it's essentially moldable sand that's perpetually moist. You can purchase it but we found a easy homemade recipe that is more cost effective over at Skip To My Lou.
You'll need 9 cups of extra fine play sand. Cindy of Skip To My Lou used colored play sand from Crayola. Next add 3 cups of corn starch and pour in 2 1/2 cups water. Start with 2 1/4 cups and add in the rest bit by bit until you get the desired consistency. Store in a covered container. The next time the kids want to play with the moon sand, add a few tablespoons of water to restore.
(pic via The Good Life)