I wasn't happy with the way the square waves sound, so I explored using other methods of generating sound waves with Arduino.
A discussion about digital sound synthesis.
An R/C servo can be used to make a gauge to measure the intensity of light.
Exploring the signals behind Arduino's Servo library, and characterizing a servo's response to pulse width.
Set up an RGB LED to phase through the colors of the rainbow, or use twist knobs to make a custom color.
Characterizing button switching behavior is an important first step for developing effective debounce algorithms in your systems.