Today is National Public Lands Day and there are activities around the country that will help you participate in you public land near you.
Try the National Environmental Education Asso.'s site with its interactive map.
Try the USFWS to see if they have something going on.
Find a complete list of Federal Partners here.
Can't find anything official? Go to a public land: local, state or federal and spend the day playing in your favorite way, biking, fishing, hiking reading, just go and play. Pack out more than you packed in. Have fun!