Harry might encounter some magical nomads. These are wizards and witches who don't settle in one place but travel around the wizarding world. They could introduce Harry to different cultures within the wizarding community and show him some unique magic that he has never seen before.
He might meet some old retired wizards who can tell him stories about the old days of magic. For example, they could share tales of how Hogwarts was founded or the great wizarding wars long ago.
Well, perhaps Harry would go to a wizarding beach where he could practice magic without the usual Hogwarts restrictions. He might meet some new magical creatures that are only found near the sea.
He could meet other young wizards and witches who are also on vacation. They might be from different parts of the wizarding world. For instance, a group of wizards from a far - off land with unique magical traditions.
In many Harry Potter summer vacation fanfictions, Sirius Black is also a character that shows up. Given his relationship with Harry and his status as a wrongly - accused wizard. He could be involved in protecting Harry during the summer or helping him with some secrets about his family. Luna Lovegood might also be included, with her unique perspective on magic and the wizarding world, she could add an interesting element to the summer vacation stories.
One common theme is discovery. Harry and his friends often discover new places in the wizarding world during their vacations. Another theme is friendship. Their relationships are strengthened as they go through different adventures together. And there's also the theme of magic outside of Hogwarts. We get to see different types of magic being used in various vacation settings.
He might meet Canadian witches and wizards. There could be a friendly wizard who shows him around the Canadian wizarding community.
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 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 could meet his future children, if he has any. They would likely be powerful wizards or witches with their own adventures. Also, he might run into the future versions of Hermione and Ron. They would probably be in important positions in the wizarding world, like Hermione might be a headmistress at Hogwarts or something.
He might meet the future versions of his friends like Hermione and Ron. Their children or grandchildren could be there, and they might have different personalities and magical skills compared to their predecessors.