Light a bulb or an LED with some Lemons, Nails and Pennies/Wire.
Some Lemons (fresh and juicy are best)
Some Galvanized Zinc coated Nails.
Some copper pennies, wire or pipe.
Wires to connect things together.
Multi-meter if you have one.
As per http://www.quantumbalancing.com/news/lemon_battery.htm
Basically the Lemon provides the acid for a voltaic battery where chemical energy is turned into electrical energy. The Zinc and Copper react with the acid to lose and gain electrons respectively. When part of a circuit this results in a current that can be used to light a bulb or an LED (dimly).
The Zinc nail and penny are electrodes. Here the Zinc nail is the Cathode (negative terminal) and the copper as the Anode (positive terminal) of the battery.
You will need to connect several in series to produce enough current to light the bulb or LED though. No resistors will be needed here if you use an LED.
Combine this with making a switch
Put the multi-meter in the circuit as an Ammeter to show the flow of current
What happens to the current when the circuit is broken by the switch being off?