If you live in an area with water or wetlands, you can buy cheap water testing kits from hardware stores and test the water-- research the kinds of bacteria that bodies of water need to be healthy and look for possible chemical contamination. I did something similar as a kid, except I tested soil around various bodies of water using cheap soil testing kits found at Home Depot.
The student can do a little background research and display their findings about the health of the ecosystem in the area in which they live. Research ways to improve the health of the local environment or have the student look into various sources of contamination. It's still fairly easy for a kid that age, but also looks impressive and thoughtful.