For contaminants, you need to explore how it gets there, how it moves, why it's a problem with examples, and solutions. For many of these topics, I suggest starting at EPA

For educational activities, you need to have an activity developed including all materials.

Coral Reef Degradation: Problems and Solutions - Ken Patterson
Dioxin
Cryptosporidium - Randi Jackson
Disinfection by-products - Erin Casperson
MTBE
Brownfields - Patricia Scott Hill
Innovative treatment technologies for air or water - Diana Stryker
Exotic aquatic species including control - Laura Carlson
Asbestos - Linsey Smith
Acid Rain - Dustin Anders
Viruses - Amanda Conally
Stormwater - Robin Stevens
Radioactive waste - Desiree McMillan
Environmental Justice
Development of environmental curricula activity
Local watershed health/neighborhood issues
Effects if increased CO2 on health - MaryAnne Summers