This visual example of erosion would help students understand how human activities can affect the landscape. One bottle could represent an untouched forest, another could represent an area with houses built, and another bottle could show land cleared for farming. The students would be able to see how humans can negatively impact Earth's natural processes.
Marine Oil Spill Lab: Great inquiry lab to get kids thinking about the environment, problem-solving, and how much more difficult it would be to clean up a real oil spill. Website has as detailed teacher instructions as well as a downloadable copy of the lab.