We spotted this sandbox over at the Cookie Magazineblog where there's more outdoor inspiration worth checking out. We like the idea of the sandbox being able to move. It would make mowing around them a snap and kids would love getting to push it around to a new spot, especially if it means being able to play in the shade throughout the day.
If you make your own like the one above, don't forget about making a lid for it to keep the water out. It could even be strong enough to double as seating or a place to put an outdoor buffet if company is over.