A diary merely put's memories onto paper.
Dear Diary I did this and this today ...
I believe the important thing is to 'think' and recreate past experiences.
Experiences are the memories that have impacted you. Something you do not forget with the passage of time. Your first kiss, your first car, your first girlfriend.
They all tend to share a common characteristic. They are tied to some strong emotion.
This is why we tend to forget a great deal of incidents in life .. no emotional tie to them or at least not a conscious one.
If you can recreate experiences tied to emotions ... those are the ones to document in the dear diary!