If you're looking for a simple project that's fun for a beginner - consider the "Annoyatron". Just like the old cricket projects from many years ago, this uses an Arduino board that plays noises when it is in darkness. When the lights come on the sounds cease - just like a cricket. Oddly enough the designer used an LED as a light sensor, however it works with some calibration. The key to success will be hiding it effectively, and in this example they've used a tissue box:
By fitting it inside a tissue box - the hardware will be protected once discovered and possibly thrown at the creator. So take care! For more information visit the project page here. And we're on twitter and Google+, so follow us for news and product updates as well.
It can be used as a noise-maker driven by your microcontroller for audible feedback of events, and it can also be used as a knock-detector input to sense events and react to them. Includes a built-in 1M resistor to allow the piezo element to detect shocks. For more information and to order, please visit the product page here.