Clothes Pin Light and Sound
Using a clothes pin morse code key (or another other on/off switch, light up an LED and send a tone to a buzzer or speaker.
Build a clothes pin CW (morse code) key as illustrated at http://mypetarduino.com/MorseCode1/codeOscillator.01.html
Build out the circuits for a buzzer and LED as illustrated, such at when the clothes pin (or any CW key) contacts connect, the LED lights up and the buzzer sounds (pitched defined in the Arduino code).
The Arduino code is adapted from their example "Button" sketch (see comments in the code for full credits).
Once built and software uploaded, you will have a morse code practice setup.
If you are a licensed amateur radio operator, you can use it to send audible CW on the appropriate bands for your license.
Or you can practice sending CW with this, or just show off your CW skills with your friends and let them try it!