As with Action Scuba, we are a 100% AWARE dive store, during dive #4 of the PADI Search & Recovery specialty I always bring my students to pick up trash as part of the performance requirements.

Instead of having them perform a search pattern in order to recover different object I would have hiden, I ask them to perform a search pattern in order to clean an area of the dive site. To complete the performance requirements, there's always something big enough to use the lift bag!