You would used the plural 'adventures' if a book had a different adventure say, in each chapter, so that it is a book of adventure stories.
"Adventure" in the singular is as with an Indiana Jones film, where it is one story filled with peril, thrills, and excitement.
Here, you are talking about a "journal" and what happens in her life from week to week. Since different situations would arise, and these apparently are exciting and entail some hazzardous thrills, then the plural 'adventures' would be preferable.
