Maybe they go on a journey to find a lost artifact that has the power to control both vampires and demigods. Along the way, they face various challenges like werewolves who don't want them to succeed and demigod enemies who see Bella as a threat. Percy would use her strategic thinking from her demigod training, and Bella would rely on her instincts from living in the vampire world.
One possible adventure is that they are called to a mysterious place where the boundaries between the demigod world and the vampire world are breaking down. They have to work together to fix it. Percy has to deal with the mythical creatures that are spilling into Bella's world, and Bella has to handle the vampires who are getting curious about the demigod realm. It would be a wild ride full of danger and discovery.