Google the National Parks Site. I did this last year, I mean there are hundreds of National Parks, Memorials, Monuments, Historic Sites, Battlefields, etc. There are more than I ever imagined!! I purchased a Park Cancellation Book last summer where you go to the majority of the parks and they have (ink) stamps that you can use to stamp your book with the date & location. If you go to this web address, the National Park service has a state by state list of their stamp locations. Whenever we go on a trip, I always manage to hit a few of them. I don't think I will ever get to every one of them. I did't count them but they claim there are over 600 Cancellation Stations in the USA, Hawaii, Alaska, Puerto Rico, Virgin Islands.... http://www.eparks.com/store/content/863/Passport-Cancellation-Locations/
Have a Safe Trip
Bob Ralston
