I built the Toy Organ found on page 23 of Forrest M. Mims III's book Timer, Op Amp & Optoelectronic Circuits & Projects. You can buy the book at your local Radio Shack. Part number 62-101.
I added a toggle switch for power and a LED to indicate power. Mine uses a 9V battery. If I build another one, I'll add a pot for volume control and probably mount the buttons closer together or in a different layout. It would be neat to experiment with different types of switches as well, like a slider instead of a rotary potentiometer, smaller buttons with less travel in the actuator would also be nice.
It's fun to play with even though I have no musical abilities.