As shown above, I have made a speaker that is controlled by the amount of light that it receives. Bonus: I have found the perfect filter on YouTube for my project!
Once again I am using phototransistors as analog inputs, one of them controls the pitch of the sound and the other the duration. This way we get an instrument that can be controlled with both hands of the user. It is interactive and it provides hours of fun, although the square wave form will quickly give the audience a headache.