Test products for impurities. For example, you could compare the amount of lead in different brands of bottled water. If a label says a product does not contain a heavy metal, is the label accurate? Do you see any evidence of leaching of hazardous chemicals from plastic into water over time?
Which pesticide is most effective against cockroaches? ants? fleas? Is it the same chemical? Which pesticide is safest for use around food? Which is friendliest to the environment?
Can you build your own electrochemical cell or battery? Test its output and efficiency.