Thalia's Tree (Symbol)
While Thalia's tree is a real, physical thing that protects the borders of the camp, it also symbolizes sacrifice and friendship. The tree gets its power from the sacrifice Thalia made to save her friends all those years ago, and for it to be failing not only directly affects the camp, but also symbolically hurts her memory. We see this in the fight that Annabeth and Luke have, where Annabeth asks Luke how he could dishonor Thalia by hurting her tree.
The Caduceus (Symbol)
Mythologically, the caduceus is the symbol of Hermes, which is how Percy recognizes him that night at Camp Half-Blood. However, the symbol in this context represents travelers and messengers, which is why, in Sea of Monsters, it originally appears in the form of a cellphone. This isn't to be confused with the more popular use of the caduceus, which is as a symbol of medicine—Apollo, not Hermes, is the Greek god of medicine.
Trickery (Motif)
Cleverness is a theme of this novel, but specifically, we see the motif of trickery occurring again and again. While there are multiple ways that the characters get themselves out of sticky situations, often they rely on tricks to get them through. Grover, for instance, uses Penelope's old trick with a funeral shroud to avoid Polyphemus, while Annabeth pretends to be Nobody in order to distract him.
Modernizing (Motif)
Speaking of old tricks, throughout the novel, we see ancient things morphing into more modern versions of themselves, often in humorous ways. The Hydra, for example, becomes the basis for franchise restaurants, while Circe's island becomes C.C.'s spa. Since the West is constantly moving and modernizing, the myths of the Greeks are moving and modernizing as well.
The Golden Fleece (Symbol)
Like Thalia's tree, the Fleece is a physical object with physical powers. However, it also represents hope. At the beginning of the novel, it seems like an impossible thing to strive for, too good to be true. As the characters continue to pursue it, they find that it's elusive but not impossible to secure. For a minute, as they lay it over Thalia's tree, it even seems that they've captured it permanently. But as often happens with wishes and hopes, it the results aren't exactly as expected. While their hopes for Thalia's tree are exceeded, it also brings unforetold consequences.
