He could meet the main characters from Death City. For example, Maka Albarn and Soul Eater Evans. They could be initially cautious around Harry but then become friends as they realize his good intentions.
Potentially, Harry could meet Stein, the mad scientist - like character. Their meeting could be full of scientific and magical discussions. And then there are the other meisters and weapons in Death City. Harry's presence could disrupt the normal balance in the city, and these characters would have to figure out how to deal with him, leading to all sorts of interesting character dynamics.
Sakura might be one of the characters. She has her own set of skills in the medical and combat fields. Meeting Harry could lead to exchanges of knowledge about different forms of healing and combat magic.
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.
Obviously, he could meet Darth Vader. Their encounter would be really interesting as they are from two very different fictional universes. He might also come across Luke Skywalker. They could form an alliance, with Harry sharing his knowledge of magic and Luke sharing his skills with the Force. Another character could be Han Solo. Harry would be amazed by Han's piloting skills and Han would be intrigued by Harry's magic.
Obviously, Buffy would be a major character he'd interact with. They could start off as wary of each other but then grow to respect one another's fighting skills. Giles too, as he has a wealth of knowledge about the supernatural, and he and Harry could have some interesting discussions about different types of magic.
He could meet Yugi Moto. Yugi is a key figure in Domino City and his connection with the spirit world might be really interesting to compare with Harry's magic. Also, he might run into Joey Wheeler, who could be either fascinated or a bit skeptical about Harry's magical powers.
He could meet Bella. Since Forks is Bella's hometown in the 'Twilight' series. They might become friends or have some kind of adventure together.
He could meet a young and talented French wizard who becomes his guide in France. This wizard might show Harry all the hidden magical spots in France.
He could meet a Canadian wizard named Oliver who is an expert in ice magic. Oliver could teach Harry how to use magic in the cold Canadian climate.
He could meet the ghosts of his ancestors. These ghosts might offer him some advice or reveal some family secrets that could help him in his future adventures.
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.