I just learned about this one, so am passing it along.

America the Beautiful ? National Parks and Federal Recreational Lands Pass ? Senior Pass

* Cost: $10
* Lifetime pass for U.S. citizens or permanent residents age 62+
* Provides access to, and use of, all national parks and Federal recreation sites that charge an entrance/standard amenity fee
* Pass may be used for passengers in a non-commercial vehicle (at vehicle entrances), or for pass holder plus 3 adults at per person fee areas
* Provides a 50% discount on some amenity fees
* Non-transferable and generally does not cover special recreation permit fees
* Purchase: In person at the park