There are quite a few battle scenes, mostly sword fighting. And, most of these battles are monsters out of Greek mythology attacking teenagers. Nothing graphic, but there are some scenes that could frighten younger readers.
Sexual Content -
Annabeth and Percy kiss
Piper and Jason kiss
Annabeth and Percy are talking late at night and fall asleep sitting next to each other. Nothing happens. They're found the next morning by their friends and are made fun of for the rest of the day.
Other Unmentionables -
Jason, Percy, and Leo get possessed by evil spirits and try to kill each other. In the end, the spirits leave their bodies.