(1) Siren's Song: While sailing the Sea of Monsters, Percy encounters the Sirens, mythical creatures with enchanting voices that lure sailors to their deaths. These are the same dangerous Sirens that Odysseus faced in ancient Greek myths, and they also attract Annabeth and Grover to jump into the sea.
(2) Charybdis and Scylla: Percy and his friends must navigate a treacherous strait between the whirlpool Charybdis and the six-headed monster Scylla. Charybdis constantly sucks in water and creates violent whirlpools that can destroy any ship that comes near. On the other hand, Scylla is a monster that lives on a cliff and has six heads, constantly devouring anything that passes by.
(3) Polyphemus the Cyclops: Percy's group encounters Polyphemus, a one-eyed Cyclops, who captures them and traps them in his cave. Polyphemus devours several members of Luke Castellan's crew and threatens Percy and his friends. Percy has to use his intelligence and courage to outwit the Cyclops and escape from the cave.
(4) Lotus Hotel and Casino: Percy, Annabeth, and Grover find a tempting resort where time stands still. Everyone who stays at the Lotus Hotel and Casino has no sense of time passing and becomes content and forgetful. Percy must resist its allure and convince Annabeth and Grover to leave before they become trapped forever.
(5) Telemachus and Calypso: Percy reunites with Telemachus, Odysseus' son, who is trapped on Calypso's island. Calypso, the goddess of the island, has kept Telemachus captive for many years and offers Percy the chance to stay with her as well. However, Percy manages to convince her to let Telemachus go and continue on his quest to find Percy's father, Poseidon.
(6) The Fury Tisiphone: Percy faces Tisiphone, one of the Furies, who appears as a taxi driver and threatens Percy and his friends. She tries to distract Percy and cause him to lose focus on his mission.
These are just a few examples of the many challenges and obstacles Percy encounters during his journey back home after battling the forces of Kronos and his monstrous allies.