Can't donate a cool billion? Well, the National Parks Conservation Association says that one of the best ways to support parks is, indeed, to visit one. Go even deeper by joining a citizen science project, like in the Great Smokies (dlia.org) or SoCal's national parks (researchlearningcenter.com). Or taking one of the American HIking Society's 50 Volunteer Vacations. (americanhiking.org).