The Junior Ranger program is obviously so great.  We've also found that the National Parks Passport book is such a fun way to get kids excited about going to visit a new park- even something as simple as getting a new stamp in their books can completely frame the itinerary of a trip and give new places a whole new perspective!  We bought my daughter a NPS Passport when she was an infant, and plan to keep the tradition of collecting new stamps every summer as long as we can- it is fun for everybody and forces us to expand past our immediate area, as well as looking for smaller or off-the-beaten-path parks that we may not necessary be interested in visiting otherwise.