A sandbox is a reasonably scaled and so infinitely variable a toy that it should be an important consideration for any family with children and the use of about 20 square feet of ground.  The basic design is simple: 1) Make a collar of the depth you want out of rot-resistant material (i.e. heart redwood, cedar, et. al.) and 2) fill to desired level with sand.

That is the most obvious design consideration but not the most important factor by far.  Sand management is the important issue.  Sand is predominantly silica (aka quartz) and as purchased is fine grained and fairly uniform in size.  Thus it packs densely.  The most critical property is the extremely hydrophilic nature of silica.  It can sustain a great deal of water just by surface tension.  So, if you put sand in a blind hole on dirt, once it gets wet it can readily puddle and take weeks or months to dry, depending on where you live.  It rains sometime where most people live.  And to maximize the “toy-ness,” the sandbox must quickly drain so the children can add a dribbling hose to the party.

A design that worked for our four children:
Our sandbox is 6’ x 6’ x 1’ and is filled to 8” deep.  About 25 cubic feet of sand in all.  Although it is simple arithmetic, I’m going to give you the small numbers to inspire you.  Its position on the ground is controlled by gravity.  The mount is three layers:

1. Gravel, also called drain rock, minimum one inch deep – 3 cubic feet
2. Galvanized fencing, ¼ inch mesh, 36 square feet
3. Aluminum screen, 36 square feet
4. When not in use, protect your sandbox with a screen to keep the local cats out

Play Time:
You can also add a degree of nonsense.  I added marbles, surf tumbled beach glass and stainless flatware from the local Goodwill.  Our children would squirt water droplets into the air and the humming birds would come to collect them.  It was a four-act play that had a long run.