Buy a National Park Passport Book

National Park Passport books are another nice way of recording where you've been. They only cost about $8. The passport book breaks down the United States into different regions and lists the National Parks, National Monuments, National Historic Sites, etc. in that region. There is a book entitled The Complete Guide to America's National Parks (Serial) which lists everything under the National Park Service's jurisdiction.

Whenever you visit a national park, just stop in the Visitor Center, open to the appropriate region, and use the cancellation stamp to mark your Passport book. These cancellations record the name of the park and the date of your visit. Getting your book stamped is free.