Perhaps Percy will meet a group of environmentalists. Given San Francisco's reputation for being environmentally conscious, they could be fighting against some sort of sea - related pollution that Percy gets involved in. He might use his powers and knowledge to assist them, and in the process, make some good friends.
Well, perhaps Percy goes to San Francisco to explore its famous landmarks like the Golden Gate Bridge. He might meet new friends or encounter some challenges specific to the city.
The setting of San Francisco can add a lot of mystery. With its foggy weather, it can be a great place for monsters to hide. Also, the city's many hills and old buildings can be like a maze for Percy to navigate, making it more exciting when he's on a quest.
He could meet his old teachers again. They might be surprised to see him back after his long absence.
He might meet alternate versions of his family members. Maybe a Percy from another universe whose mother is a powerful queen instead of a mortal.
He could meet some of the older campers who usually don't interact with the new arrivals right away. They might give him some special advice or warnings about the camp.
There's a chance that Percy meets the personification of Tartarus itself. It could be a shadowy, all - powerful being that tries to break Percy's spirit. Percy, being the brave hero he is, would have to resist and find a way to either outwit or overpower this entity in order to continue his journey in Tartarus.
He might meet his former classmates who have changed a lot since he left. Maybe some new students who are intrigued by his reputation.
Perhaps there's a special event in San Francisco that Annabeth wants Percy to be a part of. It could be a celebration, a reunion of some sort, or an opportunity for them to have some time away from their usual troubles and just enjoy each other's company in a new and exciting city.
He could meet some Aboriginal heroes or spirits. They might have their own powers and knowledge that Percy can learn from.
There could be a school janitor who is actually an old demigod who retired from the hero business. He could give Percy some valuable advice based on his own experiences. And of course, Percy will surely meet Annabeth's classmates, some of whom could be friendly and some not so much, depending on how they react to the new boy in school.