How to build a sandbox - (article with good simple instructions)

I am so antsy to build a puppy playground in the back yard (if only our darned landscaper would finish the landscaping already) and saw a show, "Ask This Old House," where they showed how to build a sandbox.

I've been looking around for a sandbox to buy, but everything I've found is too little (meant for little bitty kids) or too expensive.

This show segment made me decide to build one myself! I Googled to see if this project was online, and found it here in case anyone wants to build one for their pups:
http://www.thisoldhouse.com/toh/how-to/intro/0,,20176015,00.html#

My puppies love digging so much, and they're good about staying out of places they shouldn't dig, but I really want to give them a "safe" spot for digging.

BTW - the other things in my puppy playground include:

- four logs, laid out so that they can hurdle-jump over them (they LOVE jumping)

- a ramp (plywood propped and secured to a large log) (they love running up and down things)

- an 18' agility tunnel (they love running through things)

- the sandbox (with some Kong-type toys that I will bury in the sandbox for them to dig up)

- maybe a few upright items they can run through/around (they think our front porch is a playground - they love chasing each other underneath/around the rocking chairs, but they're getting a little too big to fit underneath)

Any other ideas for fun stuff for the pups? I may include some other agility equipment (just for their enjoyment... I don't think I'll get them into formal agility competition or anything). The whole playground will be mulched (if I can find a pet-safe mulch).