A couple of ideas from projects I worked on as a kid:
Get extracted teeth from a dentist and soak them in different forms of fluids to show the decay process. Interesting point, soda pop will completely disolve them away. Other things like juice will stain. He should take them out daily and brush them as to simulate a hygeine process. That's a pretty interesting one.
Something else that's made headlines in the last few years are a couple of different kids in cities who went around town and got sample of ice from public ice machines - McDonalds, the gas station, 711/convenience stores, an airplane if possible, etc. And sampled them under a microscope and identified forms of bacteria. Basically, the ice is infected with all kinds of terrible bacteria. Totally gross. So gross in fact that to this day I will not accept ice on an airplane. Maybe he can twist that by doing something similar by taking swab samples of public door handles or something else everyone touches or uses without thinking of its cleanliness.