The Earth Day theme - Walk, Bike, Row to an Earth Day Event Around the World - exemplifies ways to have fun yet help the environment. Earth Day is always April 22; it's on a Tuesday in 2008. If you can't play hooky, there are lots of Earth Day activities and trips on surrounding weekends.
Earth Day promotes awareness of environmental issues worldwide. Many adventure travelers know first-hand about the earth's fragile ecosystem. Here are websites listing hikes, activities and ways to help clean-up wilderness areas, jungles, mountains and other locales we've explore by foot, pedal power and other means.

FriedlandClick on a specific country on the map on the Earth Day Network website to find local activities. In the States, you could help with a restoration project at The Nature Conservancy’s Disney Wilderness Preserve, or collect seedlings from a gas pipeline right-of-way in West Virginia that are destined to be mowed down, and prepare to transplant them in Cranesville Swamp Preserve.

L FriedlandDuring National Park Week, April 19-27, there are many hikes and activities on Earth Day, and the weekends surrounding it. At Yosemite National Park, for example, family nature walks, a photography walk tracing Ansel Adams steps as he photographed in the park, and many other activities are planned April 18-20, and on Earth Day, April 22.
A Richmond Audubon Nature Guide will lead this canoe trip on the James River, April 19, where participants will have a chance to see eagles, blue herons and egrets.
Lots of hikes, bike rides and even climbing trips are scheduled in eastern states, including Massachusetts, Connecticut, New Hampshire and Rhode Island on Earth Day and later that week. Click on this link to find a complete list.
The Nature Conservancy has links to places in almost every state, where you can volunteer to help clean up protected reserves, which are popular places for hiking, bird watching and family walks. If you can't get outdoors to volunteer, donating to $1 (or more) to the "Plant a Billion Trees" effort is another way to help celebrate Earth Day.