Maybe he discovers some long - forgotten family secrets. It could be something about his parents or the history of the Potter family that has been hidden away in the manor.
There could be some old family friends who come to see Harry at the manor. These friends might have known his parents well and can share stories about them. Also, it's possible that he meets some local witches and wizards who are associated with the Potter family. They could offer their help or give him warnings about the upcoming threats. And perhaps there are some magical beings like fairies or sprites that have made the manor's gardens their home and Harry encounters them during his stay.
He might meet the house - elves that have been taking care of Potter Manor for generations. They could show him around the manor and tell him stories about his family.
In this fanfic, perhaps Harry might discover hidden secrets of Potter Manor. He could find old family heirlooms or secret rooms that were unknown to him before.
Well, perhaps Snape would be even more surly during the summer. He might assign Harry extra potions work as a form of punishment or to keep him out of trouble. Harry, on the other hand, would probably be more rebellious in a fanfic scenario during summer, as he's away from the strictures of Hogwarts. Maybe he'd try to uncover some secrets about Snape that he suspects.
In a fanfic where Harry Potter goes to summer camp, it would be an interesting mix of the wizarding world and the typical summer camp activities. He would likely struggle at first with not being able to use his magic openly all the time. But he'd find ways around it. For example, he could use a levitation charm subtly to help set up tents faster. He might also meet other young witches and wizards in hiding at the camp. Together, they could explore the secrets of the camp, like an old abandoned cabin that has some magical properties. And he'd have to keep an eye out for any Death Eaters or their followers who might be lurking around, trying to cause trouble for him even outside of Hogwarts.
Hermione would probably be a bit worried at first. She'd be concerned about him missing out on important magical studies during the summer. But she'd also encourage him to have some normal, non - magical experiences. Ron, on the other hand, would be jealous. He'd wish he could go to a muggle summer camp too and experience something different. He might even ask Harry to bring him back some muggle treats.
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.