American Hiking Society

American Hiking Society is a nonprofit that wages a never-ending battle against degradation of our nation's hiking trails. To that end, it offers one- and two-week volunteer vacations, during which its participants pull weeds, remove fences, dig trenches and perform other useful tasks in some of America's loveliest national parks. Like the other organizations we've listed, this idealistic group partially relies on donations from both individuals and the corporate world.