For something fun and different, consider building your own Theremin. This is a simple device that generates a radio wave and send the frequency to the Arduino to be converted to a simple sound matching the frequency. The term "theremin" comes from the effect that occurs when a person (or conductive material) comes close to the antenna (shown below) and causes the frequency of the oscillator to change.
For more information, complete plans and a video demonstration, visit the project page here. And we're on twitter, so follow us for news and product updates as well.