Press the button to produce a constant tone from the speaker.  It's great for practicing Morse code.  Close the switch and you have a police siren where the pitch rises as you press the button and falls when the button is released.

Components:  6 resistors, 3 capacitos, 2 diodes, transistor, IC, speaker, push button, slide switch, battery snap

Principles taught:  555 timer, voltage controlled oscillator