In the 70s us girls had little frilly books with flowers, complete with a small silly lock and teeny tiny key. Really tho if you lost the key or say your little brother stole it and accidentally flushed it down the toilet while trying to read your deepest darkest secrets on the pot. You could always just pick the lock with a fingernail file or just your nail it's self.
Or perhaps if you were a little more tomboy...a radical with a punk rock/ new wave style and wrote your thoughts in a composition book. No girly multi-colored pens, just a black Bic ball point. Like Patti Smith or Jim Carroll.