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.
Maybe he'll meet some of the shopkeepers in a more personal way. Not just as a customer, but they might recognize him as the famous Harry Potter. The owner of Weasleys' Wizard Wheezes, for instance, could give him some special new products to test out, seeing as he's friends with the Weasley family. And it's also possible that he meets a mysterious wizard who has some connection to his past or future, adding an element of mystery to his shopping trip.
There's a chance he'll meet some other Hogwarts students. For example, students from different houses might be there. He could meet a Slytherin student who might try to taunt him or a Hufflepuff student who could be really friendly and offer to share some shopping tips.
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 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.
Harry could meet some local magical creatures that are unique to Africa. For example, there could be a magical creature similar to a hippogriff but with different characteristics. And he may also meet African witches who are experts in herbal magic, which they use to heal and protect their communities. They could teach Harry some of their techniques and knowledge.
He could meet Fleur Delacour's family. Since Fleur is from France, her family might be an important part of the French wizarding community that Harry interacts with.
He might meet international aurors. These are highly skilled wizards from different countries who are experts in dark wizard catching. They could teach Harry some new combat spells.
He might meet Canadian witches and wizards. There could be a friendly wizard who shows him around the Canadian wizarding community.
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.
He might meet other Hogwarts students who he doesn't know very well. They could become good friends through their shared love of Quidditch.
He could also meet some sort of dark wizard or villain that is even more powerful than Voldemort in this new world. This would pose a great threat to Harry and force him to use all of his skills and knowledge, both from his own world and what he's learned in the new one, to defeat the enemy.
He could meet an Egyptian wizard who is an expert in ancient magic. This wizard could teach Harry a great deal about the history of magic in Egypt.