There was a man in the late 1800's who had a theory. You could put a man in a metal gage and apply as much voltage as possible and the man would survive. This is true, as long as you are not connected to earth. I showed my kids this by getting a transformer. Had one grab one wire, then some people hold hands and grab the other wire. Then I applied a voltage with a double A battery. They had 6,000 volts running through them, didn't even know it. Until I collapse the circuit by removing the wire from the battery. That's called induction. The amps that a AA battery produces is low. Milli amps, which is current and is what k_lls. Kind of like bird on a wire. Ever wonder why they don't get hurt? Cause there body is at the same potential as the wire. 18,000 volts. Anyways, I think that can be cool. Another suggestion to prove this point in comments. It's an A.
eggs - how to tell if raw or cooked without cracking; how to tell how old an egg is by "floating" it in water...eggs are marvelous! I won't tell you the answers to those science ideas, but you get my drift.
Make a model of the cell out of different colors of clay. To really make it awesome you could make it out of something edible (different type of candy on top of round, frosted cookies for example) and bring supplies for your class so they can make their own too (brownie points).