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.
Hagrid could be a great help. He has a lot of connections in the wizarding world, especially with those related to magical creatures. He might introduce Harry to someone who needs help with taking care of or studying magical beasts, and thus get Harry a job in that area.
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.
Harry, Hermione and Ron are staples. But don't forget about Fred and George Weasley. They could be at the summer camp causing all sorts of mischief and coming up with new pranks. Seamus Finnigan might also be there, with his penchant for accidentally setting things on fire with his magic. And then there are the teachers. Hagrid could be at the summer camp, introducing the students to all sorts of magical creatures. Snape might be there too, though still as grumpy as ever, teaching potions or some other dark arts - related subject.
One interesting story could be about the characters going on a magical adventure during summer break. Maybe they discover a hidden wizarding beach with unique magical creatures. Another could be a story where they attend a special summer school for advanced magic, facing challenges and making new friends.
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.
One common theme is friendship. Since summer is a time when students are away from Hogwarts, it often explores how Harry, Ron, and Hermione maintain their friendship during the break. For example, they might have adventures in their hometowns together.
Dumbledore could be the instructor. He might have a special training program in mind for the students. He could teach them about the deeper mysteries of magic, like the secrets of the Hallows or the true nature of the Deathly Hallows. His wisdom and experience would make him an excellent teacher for the students during the summer training.
In a 'Harry Potter Summer Training' fanfiction, perhaps the training is organized by some of the older students who have graduated but still want to help. They could be teaching the younger students advanced dueling techniques. Harry might discover a new aspect of his magic during this training, something that has been dormant until now. For example, he could have an affinity for a type of magic that is rare, like time - related magic. The story could then revolve around him exploring this new power under the guidance of the older students.