http://www.publiclandsday.org/
Quoting:
National Public Lands Day is the nation's largest hands-on volunteer
effort to improve and enhance the public lands Americans enjoy. In 2005,
nearly 90,000 volunteers built trails and bridges, planted trees and
plants, and removed trash and invasive plants. Join us Saturday, September
30, 2006 for the 13th annual National Public Lands Day and help us care
for our land. We invite everybody from federal land management agencies to
state parks and playgrounds in local neighborhoods to participate.
See the website link above for more information including a calendar of
events (in the Get Involved section).