My 3 children had a great time when we did this 2 years ago.
The passport cost about $10 and includes a EPCOT button and stamps from each country.
Each country in the World Showcase will rubber stamp their page and write something in their own language, usually Hello or some other kind words. The kids (15, 12, and 8) really enjoyed going to each country to have their "passports" stamped. And we were able to enjoy visiting each country.