The big decision is where or when in time to go back to:

Go back to that fateful Halloween night and save his parents?

Or back to rescuing and staying with his godfather in Third year?

Or Fourth year where he discovers the fake DADA teacher?

Or Fifth year where he contacts Sirius before rushing to the Dept of Mysteries?

Or Sixth year and stops Malfoy and that cabinet and / or saves Dumbledore?

Would Seventh year be too late to do anything to effect the outcome of the war?

Decisions, decisions, and more decisions for you to write about.