The National Park Service needs people to work with visitors, publications, research, history, cultural events, archeology, geology, natural science, hydrology, and preservation.
You can track your volunteer hours and receive a National Parks pass to visit national parks free.
According to the NPS:
Scores of NPS employees began their career as a volunteer in a park. There are many sources of information regarding such opportunities today, including many park-specific internship programs. Simply visiting a park may be a chance to ask about volunteer opportunities. Volunteering gives you a chance to get experience, and get a taste of what it is like to work in a park. Because all parks are different, each park will give you a different experience with a different set of people. As with any job, it is a chance to network and find out what everyone does in a park and how those jobs relate to one another. Central office experiences give an overall big-picture by which to navigate a career.
You can definitely put the hours you volunteer onto your resume; it depends on the government agency whether volunteer hours will be acceptable versus employee hours.
In 2005, over 137,000 volunteers donated 5.2 million hours to the national parks at a value of $91.2 million.
Volunteers come from all over the world to help preserve and protect America's natural and cultural heritage, and volunteers of all ages lend their time and expertise.