With the environment getting more coverage these days, due in part to heightened interest in climate change, Earth Day may be a big draw for your audience.
Events are on tap for communities large and small across the country. Earth Day itself is April 22, 2008, but it will be celebrated in many places on Saturday, April 19. One starting point for finding local events is the Earth Day Network's Web site.  You can search by location, keyword, or date. There also are links to many related events such as voter registration, religious group activities, climate change, and international efforts.
This list is by no means comprehensive. For other events and activities, check with local organizations such as schools and universities, museums, environmental groups, government agencies, political party organizations, and companies seeking to boost their profile.