Passport to National Parks is a great program that adds interest to the national parks visits. Every time I go to a new place I am checking to see if any stamp locations are nearby. It pays to be prepared before you go. Combining online resources with the information in the Passport Guide can save hours spent driving in circles looking for possible locations.
Check out these National Parks resources
National Parks Service site contains the information about the parks, hours of operation, maps, phone numbers, sites and history. I store all this information in Evernote and have it available when on the go.
Passport® To Your National Parks site. Here you can purchase variety of related products, but most importantly there is a link to the Passport Cancellation Locations that can be downloaded as .pdf to Evernote.
Organizing cancellation stamps collection in Evernote
This is the fun part. Using the park info and the scans of the actual stamps I created a shared Evernote notebook with all the parks I visited as separate notes.