Well, he might face new monsters. For example, some ancient and more powerful versions of the ones he's already fought. Maybe there are new quests waiting for him at the camp, like finding a lost artifact that has the power to either save or destroy the camp.
Another possibility is that there are internal camp politics or mysteries that Percy gets involved in upon his return. There could be a group within the camp that has some secret plans, and Percy, being the hero he is, has to figure out what's going on and stop any potential threats to the harmony of the camp.