One adventure could be that they find an ancient, lost spellbook. But this spellbook is guarded by magical traps. They have to use their combined knowledge of magic, Ron's strategic thinking, and Harry's bravery to get through the traps and unlock the secrets of the spellbook. Along the way, they might learn new spells that help them in their journey to clear their names or fight against whatever made them run away in the first place.
They could have an adventure where they stumble upon a hidden magical creature sanctuary. They might need to help protect it from poachers or some other danger.
Maybe they end up in a Muggle town by accident. It would be really funny to see how they try to blend in and use their magic without getting noticed. They could get into all sorts of comical situations like accidentally making things float in front of Muggles or trying to use magic to fix Muggle technology. And all the while, they are also on the lookout for any threats from the wizarding world that might be following them.
Well, it could be about them running away to avoid some dark magic threat at Hogwarts. Maybe a new dark wizard has emerged and they don't trust the school's ability to protect them at that moment.
Well, they could embark on a quest to find a powerful magical artifact that could help in the fight against Voldemort. Sirius would use his skills as an Animagus and his knowledge of the wizarding world's hidden corners, while Harry would contribute his own unique magic. Along the way, they would face various challenges, like dark wizards sent by Voldemort to stop them, and have to outwit magical creatures protecting the artifact. It would be a great adventure filled with danger and discovery.
In France, Harry could end up at Beauxbatons Academy. There, he might participate in their unique magical competitions. He could also learn about different forms of divination or potions that are specific to the French curriculum. He might face some challenges with the language barrier at first, but as he delves deeper into the French wizarding community, he starts to pick up the magical French terms and use them to his advantage. He could also form a bond with a French witch or wizard who helps him navigate this new world.
In fanfic, one reason could be that he might feel constantly in danger at Hogwarts. With all the dark forces targeting him, he could think running away is the only way to keep his friends safe. For example, if Voldemort's followers keep infiltrating the school, Harry might feel responsible and decide to leave to draw the danger away from others.
One challenge could be finding a safe place to stay. The wizarding world is not that big, and there are many places that are easily detectable. They would need to use magic carefully to avoid leaving a trail for those who might be looking for them.
Maybe they run away because they are tired of the constant danger at Hogwarts. They've faced so many threats, and they just want a break. So they run to a peaceful place where no one knows them.
Voldemort and his Death Eaters would be obvious antagonists. They could be constantly hunting for Harry and Tonks as they are seen as threats to Voldemort's plans for total domination of the wizarding world.
The remaining Death Eaters could be the antagonists. Even though Voldemort is gone, there could be some who still hold a grudge against Harry and want to finish what their master started. They might track Harry and Ginny down as they run away, creating dangerous situations for the couple.
Maybe some jealous students from Hogwarts or Beauxbatons. They don't like the idea of Harry and Fleur being together and running away to have an adventure. So they try to sabotage their plans, perhaps by spreading false rumors or even using minor hexes to slow them down on their journey.
Perhaps he uses the island as a secret training ground. He could practice advanced spells away from the prying eyes of Hogwarts faculty and students. Harry might also build his own magical fortress on the island for protection and privacy.