Try various materials to filter dirty water, then simulate an oil clean up.
Science content (2016 curriculum):
Chemistry: Physical Changes, Solutions, Mixtures and Separating (2, 4, 5, 6)
Earth/Space: Water cycle, Water conservation (2)
Curriculum connection (2005 science topic):
Earth and Space Science: Air, Water and Soil (grade 2)
Earth and Space Science: Renewable and Non-Renewable Resources (grade 5)