Probably his time at the Ninja Academy. Naruto was always getting into trouble there, but it was also where he made some of his first friends. Also, his encounters with the Third Hokage could be shown. The Third Hokage was like a grandfather figure to Naruto, and their interactions were important in Naruto's development.
Naruto has been through a lot in his own time, so he may use that experience to adapt to the past. He might start by making friends with the younger versions of people he knows. He'll also have to be careful not to reveal too much about the future to avoid disrupting the timeline. He could use his ninja skills to earn respect and find his place in the past world.
One key event could be Naruto's lonely childhood. Growing up without parents, constantly being shunned by the villagers. Madara might see how Naruto coped with that, like when he sat alone on the swing. Another could be his encounter with Iruka, who was the first to really recognize Naruto as a person, not just the container of the Nine - Tails. This moment might be significant as it was a turning point in Naruto's self - acceptance.
I would recommend 'Naruto's Childhood Redefined'. This fanfiction focuses on Naruto's early years in a different way. It adds some interesting backstories to his relationships with Iruka and the villagers. It's a great read if you want to see a new take on Naruto's past.
He might try to change certain events. For example, he could prevent some of the tragedies that occurred in the original story. Maybe he would stop the Uchiha massacre by warning the clan or finding a way to resolve the conflict between the Uchiha and the village leadership.
He would probably use his leadership skills. Naruto has grown into a great leader as Hokage. In the past, he could quickly assess situations and lead others, whether it's in a battle or in village - building activities.
A great 'Naruto from the past' fanfiction could involve Naruto going back to the time when the Nine - Tailed Fox first attacked the village. He might use his connection with the fox to stop the attack or at least minimize the destruction, and in the process, change how the villagers view him and his family.
Well, in a fanfiction where Naruto travels to the past, he could meet his parents when they were young. He could also learn new jutsu from the past ninjas that were lost over time. Maybe he'd even be able to influence the early development of the Hidden Leaf Village in ways that would make it stronger in his own time.
There are quite a few things. For one, he could teach past Naruto about the deeper secrets of the Uzumaki clan. Since Future Naruto has explored more about his heritage. He could also give tips on handling the Kyuubi's chakra more smoothly. Maybe some new jutsu combinations that he discovered over the years. And not to forget, how to better read an opponent's moves during a fight, which is a skill he would have refined over time.
Naruto could also meet the previous Hokages in their prime. He could learn from them directly and perhaps even influence their decisions. For example, he might be able to convince the First Hokage to handle the Uchiha clan's situation differently, which could have a huge impact on the future (or in this case, the future that Naruto comes from).