Check out this awesome earth day lesson/unit on Preserving Earth's Resources I used with my students for the last few years. They LOVE LOVE LOVE it. Applicable to any grade level - focuses on inquiry and investigations, as well as some mathematics concepts.
In my case, it depends a lot on the age level, but I think it's an important issue and one that resonates well with students (I recently became a foreign language rather than elementary school teacher and earth day lessons are one of the things I miss most; foreign language is a slightly more limiting platform for earth day activities).
For younger students, I usually go for the science-fair angle and have the students develop simple experiments to understand the effects of pollution. The North American Association for Environmental Education has some great resources and ideas: http://eelink.net/pages/Classroom+Resources-Directories
For older students (even language students), I tried something I was really pleased with last year, a game/variation on the prisoner's dilema to understand the social dynamics of pollution:
Each player starts out with 10 monopoly dollars/candies/some reward system you use regularly. In each round, each player can deposit up to 1 dollar in the "clean-up fund". Every deposit is secret, but the teacher announces the total in the "clean-up fund" at the end of each round.
If the "clean-up fund" does not reach a certain level by the end of the game (you might have to play with that number and the number of rounds depending on the size of the group, but i found it worked better with small groups), everyone looses all of their money.
If, however, the "clean-up fund" does reach a certain level, players get to keep however much money they have left. Think of it as "the tragedy of the commons" in a game: collective v. personal gain. In my experience, not only did students have fun but it really made them think about the impact of their decisions and the broader social context of environmental protection.
Can a group of students make the right decisions to save their lake/forest/world (whatever background story you want to use)?
A few years ago, I had my fourth-graders bring in old newspapers and magazines from their home. Then, we re-used their paper to create a door and window display about climate change (with younger kids, your display could be about something like re-duce, re-use, recycle). They cut out letters; made pictures using mosaics of tiny, cut up photos -- it was a bit messy, but it got the point across. After the activity, they were really good about re-using paper, rather than throwing it out. They even kept me on track when it came to waste!