Get a National Park Service Passport by Hopkid
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 might not think of Hawaii as having a national park but here you can get a stamp at the USS Arizona Memorial which is operated by the NPS. Take the passport with you to Maui, Molokai, and the Big Island and get stamps there as well.
