Another vote for the autograph book, and against the passport here!
My just turned 5 year old loved having the characters "Sign UP!" This was a big highlight of her trip.
She also LOVED Epcot's World Showcase and the Kidcot stations in each country - in fact - she liked Epcot better than the MK. We did buy passports for all 3 daughters, but they were a waste of money, IMHO. The masks were so much more fun, and free. The CM's would stamp and sign the sticks in each country, as well as add the decoration for each country, and Sarah loved to continue decorating her mask - she thought she had to color more in each country as well! She has 3 of these now in her bedroom - they make an awesome souvenir of her trips! Another one is in the works for November!!
The passports are small - filled with stamp like stickers - no room for creativity, and not appropriate for display.
Hope you have a great trip!