Simile Pg 9: "A red umbrella stuck up from the back of his chair, making it look like a motorized cafe table"
Rick Riordan included this sentence in the book to give the reader more detail and to allow them to make a mental picture of what Chiron looked like sitting in his wheelchair.
Metaphor Pg 296: "I was sure that any moment she would become the world's largest Milkbone dog biscuit."
This metaphor was included so that the reader may know the amount of worry that Percy has of Annabeth being eaten by Cerberus (A three headed dog that guards the entrance to the Underworld.)
Simile Pg: 285: "His question caught my stomach like a fastball."
This particular statement shows that one of the question that Percy was asked caught him off guard and he didn't know how to answer it.
Metaphor Pg 257: "It was like a hundred and ten degrees, easy, and we must've looked like deep-fried vagrants, but everybody was too interested in the wild animals to pay us much attention."
Riordan used this metaphor to allow the reader to picture what the three kids looked like when they stepped out of the animal smugglers' truck.
Metaphor Pg 291: "But the entrance to the Underworld looked like a cross between airport security and the Jersey Turnpike."
This statement was used to show what the Underworld looks like and give readers a nice picture in their head...