A few years ago, I picked up National Park Service Passports for our kids. The passports are a way to "record" your visits to national parks, monuments, recreation areas, etc. Visitor centers and park gift shops usually have passport stations where you can stamp your books with their specific stamp and it'll record the date. My kids say that I'm more excited about the passports than they are. That's probably true, but I know I'm helping them create memories and someday they'll appreciate having this record.
When we're traveling, I carry them in my purse (you never know when you might stumble upon something) but usually they reside in my car's glove box. When we were walking down the street in New Orleans recently, I announced to the family that there was no need to panic since I was carrying the passports in case we stumbled upon something. You should have heard the ridicule I endured until I pointed out the Jean Lafitte National Historical Park and Preserve visitors center we were walking by.
You can buy them online here.