You could maybe write about volunteering in an Earth Day event where you pick up trash in your area. You try to convince your friend by telling him or her why it is good to pick up trash, what negative impacts does waste have in the environment, and why picking it up helps the earth, humans, plants, and animals. Of course, then explain why it would be fun to volunteer (helping earth and hanging out together) and your feelings about the event (positive feelings). Hope this sparked and idea for you :)
