I believe that you should use "Christine's journal is full of adventures"
As there is an idiom "Full of the joys of spring", which testifies above...
Please follow this link:
Full of the joys of spring - UsingEnglish.com
If I have a sentence like this "Christine's journal is full of adventure." Do I put "adventure" or "adventures." Which one is correct?
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.