Make a cover for your outdoor sandbox with some planks and a tarp.
Get some small planks, about 3" wide and 1/2" thick, and measure them to the bottom of the outside sandbox walls. Nail them together to form a square frame. Fit a heavy duty tarp over the sandbox and down the sides to the ground, then cut it a few inches wider than that. Wrap the tarp around the frame and use a staple gun to secure it.