These are really fun and spark interest in the National Park Service system. The brainchild of a marketing genius, the purchaser can get a stamp from each of the NPS sites he or she visits. The collection of these stamps, similar to postal cancellation postmarks (which include the name of the park and the date visited) become fun to collect. It's a great way to get the kids (of all ages!) excited about going to different parks, monuments, seashores, etc. that are operated by the NPS.
The passport itself is reasonably priced and the stamps are, of course, free. Each NPS facility has a stamp available at the visitor's center. If you don't see it just ask the ranger on duty. Some (e.g., Mt. Rushmore, Cape Hatteras Lighthouse) have special stamps with a depiction of the area/monument. Great fun!
You can obtain an NPS passport at any park Visitor's Center or online at the National Park Service Store.
You can find the passport stamp at the ranger station at Shuttle Stop No. 6. There is a small box just outside the door that contains the stamp and ink pad. Further afield you can also get stamps at Yosemite National Park (approximately 30 miles north) and Manzanar National Historic Site on U.S. 395 approximately 2 hours south of Mammoth Lakes.
Favorite thing: The middle fork of the San Joaquin River runs through Devil's Postpile National Monument, parallel to the famous John Muir Trail. The easiest way to access the river is from the ranger station, where people gather for the Postpile ranger talk. There's also a small campground near the ranger station right by the river.
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