This would be a metal lockbox with a variety of switches, buttons and LEDs covering the front in a non-intuitive pattern. To gain access you would have to trigger the switches in a specific order.
The bolts holding the box shut would be controlled by one or more stepper motors which would activate when the combination is entered.
Quandary: How to power the box? If I use batteries, what happens if the batteries go dead? Should I assume an external power supply will be plugged in? Maybe to keep the power from dying, I could have the Arduino shut itself off after a minute of inactivity, with a mechanical button on the outside that could start it up again.
(Original photo via ClintJCL / Community Commons)