There's a chance he'll run into Ron. Maybe Ron is looking for some new wizarding sweets at Honeydukes, and they end up sharing some of the treats while they continue their shopping spree. And it's possible that they'll see Neville there too, who is searching for some rare plant seeds for his Herbology studies.
He could meet Hagrid. Hagrid often goes to Diagon Alley, so it's possible he'll run into Harry while he's shopping and give him some useful advice about what to buy or not to buy.
He could meet other Hogwarts students. They might be shopping for the same event or just casually browsing. It would be fun to see him interact with Hermione and Ron in a non - school - related setting like a clothes shop.
Well, he might encounter some magical items that are only available in certain wizarding shops. Maybe he'll find a new type of wand core or a potion ingredient that's super rare.
Well, he might struggle to find clothes in his size. In fanfiction, things can be a bit different from the original story. Maybe he goes to a magical clothing store where the clothes change colors based on his mood.
In this fanfiction, Harry might go to Diagon Alley to buy new school supplies. He could visit Ollivander's to get a new wand or Flourish and Blotts for some interesting magic books.
In this fanfiction, Harry might meet students from various backgrounds. There could be students from far - off magical lands who have their own unique cultures and magic. He could also run into some Beubaxtons alumni who come back to visit and they might share some interesting stories or give him some advice on how to make the most of his time at Beubaxtons.
He could meet his future self. This would be a very interesting encounter as he could learn from his future self about what he has accomplished and what mistakes he should avoid. He might also meet the future generations of his family, like his grandchildren or great - grandchildren.
Perhaps Harry will run into an old wizard who has been in the hospital for a long time. This wizard has a lot of stories about the old days of the wizarding world and gives Harry some valuable advice.
Maybe he'll run into a group of young witches and wizards who are on an adventure of their own. They might team up with Harry and together they explore the new country. There could also be a wise old witch who lives in a secluded magical cottage and gives Harry some valuable advice about the country's magic and its history.
In this fanfiction, Harry could meet ancient Aztec spirits that have been bound to the temple for centuries. Also, there may be other adventurers or treasure hunters who are also exploring the temple. They could either be friends or foes to Harry. Additionally, he might encounter descendants of the Aztec people who still hold some knowledge of the temple's magic and secrets, and they could either assist him or try to stop him depending on their motives.