Reader Crystal passed along this great deal. If you are age 62 (or older), you can purchase a National Parks Lifetime Pass for just $10. You can bring along 3 adults with you, and they can also get in for free!
This pass provides access to, and use of Federal recreation sites that charge an Entrance or Standard Amenity Fee*. The pass admits the pass holder and passengers in a non-commercial vehicle at per vehicle fee areas and pass holder plus 3 adults at per person fee areas.