If you are thinking of building a sandbox, why not go all out and build a hexagon sandbox with built-in benches. I think you should!! Anyhow, at the link you can follow along and see how to build it.
Build your own backyard boat sandpit using the free woodworking instructions available at the link. The sail will provide a bit of shade and there is even a place to sit.
Build a sandbox for the backyard. It is an easy build and children will enjoy it for many seasons to come.