Turning a lemon into a battery is a perennial classic, a project many teachers have used over the years to introduce basic facts about electricity. However, most vegetables are also acidic enough to make the experiment work. You'll need small strips of zinc and copper, and a digital voltmeter. Insert the zinc and copper electrodes into multiple vegetables, and touch them with the meter's probes. Have the kids record the voltage differences from vegetable to vegetable, or test with the electrodes close together and far apart to see if it makes a difference. Log the results, and speculate why the vegetables vary.
Return to your vegetable battery experiment. This time, turn your multimeter to its microampere setting. Revisit each of the vegetables you'd previously tested for voltage, and test each one's amperage. Ask your students whether the voltage and amperage are uniformly high or low, or if there are some variances. Have the students test each vegetable for pH with litmus paper. Log the pH for each vegetable, and do a compare-and-contrast to draw conclusions about the relationship between a vegetable's pH and the amount of current it generates.
Impedance is the term for the resistance an electrical current encounters as it travels through a substance. It's measured in ohms, and the amount of impedance found in an object or substance determines whether it conducts electricity or insulates from it. Copper, for example, has little resistance, while wood has a great deal. Test the widest possible range of vegetables with your class, recording the impedance of each with your multimeter's probes set to a consistent distance every time. Ask your class which vegetables are conductors of electricity and which are insulators, and speculate why.
To provide your class with more information, select a handful of vegetables with widely varying physical characteristics. These might include ripe tomatoes, hollow winter squash, juicy cucumbers, spongy eggplant or zucchini, or relatively dry vegetables such as broccoli. Set your multimeter to the impedance setting again, and test the vegetables in pairs. Have the students make a chart of each vegetable's characteristics, and for each pair try to guess which characteristics make it a better or worse conductor of electricity. After you've tested all the pairs, there will be plenty of data to analyze.