Maybe it could be about Harry having some sort of self - discovery journey during the summer. He might start working out, change his style, and as a result, become more attractive both physically and in terms of his new - found confidence.
Hermione could be one of the main characters. She might be the one who first notices Harry's change and helps him navigate through the new attention he's getting. She could also offer her wisdom on how to handle the situation without letting it go to his head.
Emotionally, he could have changed a great deal. If in the fanfiction he faced some personal challenges over the summer, like dealing with the loss of a friend or a new family situation. This might have made him more withdrawn or on the contrary, more determined to face his enemies. For example, if he discovered some secrets about his family that made him question his identity, it would have a profound impact on his emotional state.
He could work at Weasleys' Wizard Wheezes. He has a good relationship with the Weasley family, and it would be a fun place for him to work, helping with inventing and selling all kinds of magical pranks and toys.
Often, he becomes more self - reliant. Since he can't always count on others during the summer away from Hogwarts, he learns to solve problems on his own. For instance, if there's a magical creature causing trouble near the Dursleys' house, he has to figure out how to deal with it without the help of teachers or friends right away.
In some fanfictions, Harry might change physically. He could have grown taller or more muscular from any summer training he did. For example, if he was secretly practicing Quidditch on his own, it could have affected his physique. Also, mentally he could have become more mature. Maybe he spent the summer reading advanced magic books and this broadened his understanding of magic, making him more confident and less impulsive in his actions.
Well, physically he could change too. Fanfic writers might describe him as growing taller or stronger during the summer break. Emotionally, he might deal with his past traumas better. For example, if he had nightmares about Voldemort, he could learn to overcome them, perhaps through some new magical techniques he discovers or with the help of his friends. This could make him more resilient and less haunted by his past, which is a significant change for his character.
Hermione would be a likely helper. She's always into books and knowledge, so she'd probably be right there with Harry, sharing her insights and helping him find relevant information.
One way could be in his relationships. He might have made new friends or mended broken relationships. For instance, if he had a falling - out with Ron before the summer, during the summer months in the fanfiction, he could have worked on getting their friendship back on track.