Are you familiar with the National Park Service's "Passport to the National Parks" program?
You can purchase the passport at any NPS visitor center. The spiral-bound book resembles an official US passport but contains pages for dated stamps for each NPS site you visit.
I intend to keep "stamping away" every chance I get.
Wikipedia's entry about the program: http://en.wikipedia.org/wiki/National_Park_Passport_Stamps
This is an avid group of collectors. The website includes an exhaustive list of all NPS sites and all the stamps available for each site: http://www.parkstamps.org/news.php