In addition to just reading the book (and/or watching the movie), you can visit Seussville's Lorax Project! There is great information, plus all kinds of games & activities. There are some great classroom activity ideas here too.
Then, for a fun crafty activity, you could also make this adorable Truffula forest!!
If the Lorax isn't for you, there are so many other wonderful ideas! As always, Tip Junkie has a fabulous round-up of Earth Day crafts & recipes!. And, I have a post on Savvy Sassy Moms today with a few other simple Earth Day ideas.
What eco-friendly/Earth Day activities do YOU like to do with your kids?