Advanced Human Battery
Do you want more voltage? Switch out the zinc for a more reactive metal and get your friends in on the action. You can make a sodium-copper battery by having one lucky volunteer touch a small piece of sodium metal. Have the next person join hands with the person touching the sodium. Make a chain of human hands with as many people as you have available (the record for this type of human battery is said to be 1500 people!) and have the person on the end dip his or her finger into the copper sulfate solution. Your human battery should deliver about 3 volts.
Sodium metal is extremely reactive. Be sure to keep the sodium metal far away from any liquid water and have the person who touched the metal rinse his hand with a vinegar solution following the demonstration.