The Ice King's diary is a diary that belongs to the Ice King. It appears in "The Chamber of Frozen Blades" when Jake grabs it from under the Ice King's bed and tries to convince Finn to read it over for "proof" that the Ice King is going to steal a princess, but just wants to read it for himself.
It is a pink diary that has a golden button lock attached to it and a red ribbon sticking out of one of the pages. The front cover of the book has a butterfly sticker and a paper taped to it that says "KEEP OUT!" in red letters. On the back is a picture of Gunter and a sticker of a rainbow containing the colors pink, blue, green and yellow.
The Ice King's dairy only has one known entry. It was read by Jake in the episode Chamber of Frozen Blades, it said...
Dear Diary AND YOU BETTER WRITE ME BACK THIS TIME,
Today I'm bringing home a special new cutie.