He could have adventures like exploring uncharted magical islands. Maybe he'd encounter new magical creatures and have to use his wizarding skills to either befriend or fend them off. For example, he might come across a group of mermaids who guard a powerful magical artifact on an island.
There could be. In fanfiction, the concept of Harry Potter owning a ship can be explored in many ways. The ship might have magical abilities like being able to teleport within the wizarding world. Some fanfics might focus on how Harry discovers and uses these abilities for his adventures or to help the wizarding community.
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.
Some of the Hogwarts professors could also assist. Professor Flitwick could use his charms expertise to add special touches to the city - ship, like making the doors open with a unique charm. Professor McGonagall could use her transfiguration skills to transform materials into the needed shapes for the construction. And Neville could also play a role. He has grown into a powerful wizard, and his knowledge of herbology could be used to create a beautiful and functional garden on the city - ship.
There could be lost wizards who got stranded on the ship for some reason. Maybe they were on an expedition and something went wrong.
He could go on adventures to discover hidden vaults within Gringotts. Maybe there are vaults that have been lost for centuries, filled with ancient and powerful magical artifacts. Harry could search for these vaults, facing various traps and puzzles along the way, similar to the challenges he faced in the Philosopher's Stone.
It could be a magical sailing ship. Maybe it has enchanted sails that can catch the wind of magic and sail across different magical realms. Or it could be a small, personal ship that he uses to explore hidden corners of Hogwarts lake.
It could be a sleek, enchanted schooner. With sails that can change color according to the magical energy around it. The ship might have a figurehead that comes to life and gives advice, like a wise old wizard.
Ron and Hermione are the most likely ones. They are his best friends and have always been by his side through thick and thin. They would probably follow him without hesitation and use their own skills, like Hermione's vast knowledge and Ron's loyalty and practical skills, to assist in the ship - stealing adventure.
Well, they could have all sorts of adventures. Maybe the baby cousin has some sort of special magic that Harry doesn't know about yet. For example, the baby cousin could be able to communicate with magical creatures in a unique way. And Harry, being the curious and brave wizard he is, would want to explore this new aspect of magic with his little cousin. They might go on a journey to find a rare magical creature that only the baby cousin can sense.
Redemption is also a theme. For example, in some fanfictions where Draco is a main part of a ship, his journey towards redemption while being in a relationship with another character is explored. It adds depth to the story and the relationship.