Ride a bike!
Seriously, here are some good tips for celebrating Earth Day:
Be conscious of your energy consumption. Turn off lights/hydro when not using, don't leave water running, consider walking/biking/carpooling/riding the bus to conserve fuel. If for no other reason then to save money.
Vounteer. Most area clean-ups are done by volunteers. Consider pitching in and taking the kids.
Buy from local farms or grow veggies yourself.
Use homemade canvas bags for shopping.
Recycle!