Here's a tip-265 of your 398 national parks NEVER charge an entrance fee. So start Planning Your Visit!
If you're planning a trip that includes multiple national parks, you might consider the $80 annual pass that provides entrance to all national parks, national wildlife refuges, national forests, and many other Federal lands-more than 2,000 in all. This America the Beautiful National Parks and Federal Recreational Lands Pass is offered free to all active duty military members and their dependents. Information on these and other pass options is available online.
*Fee waiver includes: entrance fees, commercial tour fees, and transportation entrance fees. Other fees such as reservation, camping, tours, concession and fees collected by third parties are not included unless stated otherwise. http://www.nps.gov/findapark/feefreeparks.htm