Saltwater
And if you want to get technical:
Get two cups of saltwater, in one goes the aluminum, the other a harder metal such as steel, connect the two with a wire and there you have a battery.
Anode, Cathode, Electrolite. The weaker metal will corrode. I saw it done in my Painting/ Corrosion Control class for the Navy. The weaker metal began bubbling almost immediatly.