At nearly all of the 391 American National Park units, one or more National Park Passport Stamps can be acquired at no cost at park visitor centers and ranger stations. The stamps are similar in nature to passport stamps stamped in a traveler's national passport.  We picked up our books in Washington DC and we’ve been collecting stamps ever since.  The stamps serve as a record of each park visit.  They’re free, fast, and fun to collect, so if you don’t have enough time to complete the Junior Ranger program at a certain park, just get the stamp.