The Enigma Puzzle Box. Can make them as kits, Plain boxed that they ca paint or sticker up then have the wiring all pre terminated with plugs and they then put it together and make it all work.
Here's another beautiful bit of wooden puzzle design and craftsmanship from Denver metagrobologist Kagen Schaefer. The locking mechanism was recently reverse-engineered by Instructables user dombeef, who has made a working paper version.
Puzzle Box Safe tutorial(How To Make Christmas Boxes)
I've seen a few of these types of combination boxes and thought they would be great for use in role playing games such as Dungeons and Dragons.