I first saw the Passports on our trip to Yellowstone in 2010. I was too meek at that point in time to buy a passport on that trip (something that I'm regretting to this day). I thought that it was something only for kids, little did I know that there is a National Park Travelers Club for adults. John bought me my passport book the following Christmas and I was just waiting to use it until this summer when we went on our two week road trip from North Dakota to California and back.
For those of you who are unfamiliar with the passport program. In all of the national park visitor centers (and sometimes other places, as well), there is a stand that looks like this one at Devils Tower: