In fanfiction, perhaps there's a scenario where Percy and Annabeth are separated from their friends and have to rely solely on each other. They find themselves on an island full of strange and dangerous creatures. Percy uses his ability to control the water around the island to keep the threats at bay, while Annabeth studies the layout of the island to find a way to escape. There could also be a scenario where they are invited to a special demigod event that is only for couples, and they have to deal with all the relationship drama and challenges that come with it.
One interesting scenario could be Percy and Annabeth getting lost in a magical forest that wasn't in the original books. They might have to use their combined skills, Percy's water powers and Annabeth's strategic thinking, to find their way out. Another could be them facing a new monster that is immune to their usual attacks, forcing them to be creative in their defense. And they could also have a moment where they share a really deep conversation about their future together, away from all the chaos of their quests.