Each year, the National Park Service offers several days where the entrance fee to the park is free for everyone. In 2016, they have chosen 16 different days where visitors can enter without a fee. One (January 18) has already passed, but that still leaves 15 days for this year. If you visit this web page, you can read more about it. Maybe I will have to head west this August and revisit some of the parks I went to 2014.
Another thing I noted while reading the information is that any fourth grade student in the US can get a free park pass for that school year. For information on that, see the website here.