Next week is a great opportunity to viit the National Parks for free! According to the National Parks web site, "To make it easier to experience America's Great Outdoors, the National Park Service is waiving entrance fees, so visitors can enjoy all 392 national parks for free April 17-25. In addition, many national park concessioners are offering special promotions that are certain to add to your fun."
Also I wanted to mention that if you have a physical disability and have a placard, many of the national parks let you in free with the National Parks and Federal Recreational Lands Pass – Access Pass, just ask for one on your next visit to the park.
If you are a senior citizen over the age of 62, you can get an annual pass for only ten dollars. See the previous link for more details on all the annual pass options including the opportunity for anyone to get a free pass for volunteering.