Fan-Preferred Couple: Sadie/Anubis seems to be considerably more popular than Sadie/Walt. It eventually settles as a compromise of sorts when Walt is chosen to be Anubis's host.
Fridge Brilliance: In The Red Pyramid, Thoth only agrees to help Carter and Sadie if they can prove that they're not going to just repeat the old stories over and over. Except for the times where they deliberately invoke the myths, such as repeating Ra's journey through the Duat, the Kane siblings don't repeat those stories as much as the Greeks, the Romans, or the Asgardians.
Given he's the embodiment of chaos and destruction, and they specifically mention it's happening during the recent Egyptian revolution, this is true whether he personally crushed them or not.
Growing the Beard: While some agree that Riordan did this sometime during Heroes of Olympus, some also think he did it somewhere during this book, usually "Serpent's Shadow".
Hilarious in Hindsight: In Throne of Fire, Set refuses to kill a monster for Carter and Sadie even after he promised to help, on the grounds that if he did, GETM (Gods for the Ethical Treatment of Monsters) would have his hide. Come The Trials of Apollo, it turns out that the Greek gods at least banned the slaying of endangered monsters.
Tear Jerker: Bast's death.They fail to rescue their dad, they've lost practically everything, and Carter's love interest was a statue.
Actually their dad knew he wouldn't live but would be with his wife, they still have Amos and Bast, and Carter's love interest is thus still alive but sleeping, so Bitter Sweet Ending.
And then Bes sacrificing his ren-and with it, his mind-so that the siblings have enough time to beat the Big Bad of the second book.