For a fun project to share with others, that results with a useful device for children or practical jokers, consider making the "super secret lock box" devised by Reid Clymer. It's a small box that can be used to hold private items - that has two ways of opening the electronic lock (which is a servo controlling an internal latch).
You can either use a three-barrel combination method which consists of three potentiometers read by the Arduino, or if you've forgotten the combination - another clever method has been devised. This involves the use of capacitive sensing and allows a bypass of the combination. Finally you can see a quick demonstration in the following video:
With the use of the old box, the project really has some appeal, so to recreate your own visit the project Instructable. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
If you're looking for an Arduino Uno-compatible board to make your own box with, choose what tens of thousands of others have done and use our Freetronics Eleven - the Arduino-Uno compatible with low-profile USB socket, onboard prototyping space and easy to view LEDs: