Just one correction to open mike's post: The U.S. Forest Service is administereed by the USDA (U.S. Dept. of Agriculture). It's the National Park Service that is under Interior.
The park service has sites that interpret everything from soup to nuts, to coin an old phrase. ML King Jr's birthplace, Eugene O'Neill's house in Danville, CA, New Bedford Whaling, Women's Rights, and New Orleans Jazz NHS are just a few of the over 200 historical parks, sites, and monuments, not to mention all the nature parks. Presidents, artists, explorers, soldiers, and jazz musicians are all honored through the parks (a John Coltrane NHS is currently being reviewed as a possible future site). Even if you think you're too old to begin, GUEST, there are all sorts of ways to get involved as a volunteer, and many retirees/older folks volunteer in parks, as well as getting hired. There's a world of sites out there waiting for enthusiastic people to get involved in. Happy hunting, and post whenever you want more info from any of the mudcat ranger contingent.