Out of a computer can get for free: wires, motors, magnets, speaker, LEDs, battery, switches, fans.
One of main ideas of this instructable, is to use everywhere the two-wire connectors. Such connectors are widely used in radio controlled models and microcontrollers like Arduino. They allow to quickly interchange the parts and try different combinations like:
1) CD player motor as a dynamo to turn another motor.
2) CD player motor as a dynamo for a single LED or several in paralell.
3) CD player motor + loudspeaker.
4) CD player motors can be used in hobby to make laser show setup or robots, etc, etc.
5) 3V lithium cell +LED
6) 3V lithium cell +LED + magnet+tape --> throwie
7) Battery + motor
8) Battery + speaker --> click
9) 9V battery can be used to spin a fan, but 3V is not enough
10) One of further projects is to build a lighthouse with LEDs. Can add a motor spinning a small mirror cut out from a CD.
11) And something without electricity too: Magnets from speaker and HDD epoxy glued to cut-out integrated chips --> magnetic pins for a fridge.
12) More advanced thing to try: bycicle dynamo + step up transformer + diode bridge+ 300V electrolyte from power supply --> high voltage source --> sparks
13) Solar panel can be introduced too to spin a motor, make LED to shine, charge electrolitic capacitor, operate pocket calculator, electromechanical clock or a small radio.
Attached is a file how to make electric circuits from everyday materials. Unfortunately, I have written it in German as school children here speak German.