Here's a smart tip from Parenthacks: turn a wheeled under-bed storage box into a mobile, lidded sandbox that you can roll into the garage on rainy days and out into the yard on nice ones:
My husband came up with a great idea for a small sandbox: an under-the-bed storage container. This container has a hinged top but the hinge broke so I put it in the garage for another day. My husband filled it 1/2 way with play sand and our daughter loves it. We can play with sand in the garage (if it is raining outside), on the back porch or take it to Grandma's for the afternoon. When we are done we just put the top over it and slide it under the stand that holds the yard tools or under the workbench.