I'd recommend having students complete How Big Is Your Carbon Footprint? I needed to explain differences between UK and USA to my students. (Nice unplanned geography lesson.) My favorite comment from students was "There's ONLY one!" This came as a result of students sharing what their carbon footprint is as a percent. Every time a student's number was over 100%, we heard the comment repeated by multiple classmates.
My favorite Earth Day activity for students to participate in is definitely the Buncee Buddies Project.
Here is some more detail: "Buncee is excited to host Buncee Buddies: Earth Day 2016, which aspires to connect classrooms (i.e. Buncee Buddies) from around the world to explore how they celebrate Earth Day on their side of the globe! Teachers worldwide are encouraged to sign their classes up for the project and create buncees about the importance of environmental protection, methods of execution, and what the local environment is like in their corner of the world. Once you’re signed up, we will partner you and your students with another class to digitally “meet”, brainstorm ideas, and exchange your students’ creations!"
Registration has already passed but it's something to bookmark for the future.