In what doubles as a Crowning Moment of Heartwarming, the scene when the Archive notices Mister, cries "Kitty!" and goes to pet him. Living repository of all humanity's knowledge? Maybe. Seven year old? Without a doubt.
The funny part about it wasn't so much what Ivy did, but more Kincaid's reaction:
Kincaid: Now, that's just creepy.
Harry's reaction to Molly's strategically shredded school uniform and facial piercings:
"I had a vague memory of being that ridiculous at one time. Let he who hath never worn parachute pants cast the first stone."
The entire following conversation is pretty funny, too:
Molly: "Oh, so those are fun-time handcuffs, not bad-time handcuffs!"
Harry: This conversation has become way too bleeping disturbing.
Harry: Speed of lightning! Roar of thunder! Fighting all who rob or plunder! Underdog!
The "Asteroid Dresden" Brick Joke at the end had me literally rolling with laughter for a good couple minutes.
Harry summons an oracle spirit...into a Cabbage Patch doll. She is not pleased.
Harry's first conversation with the Russian Knight of the Cross Sanya, who remains an atheist despite Archangel Michael descending from Heaven to give him his sword. After Harry pokes holes in both this excuse and after Sanya amends that he may be agnostic, Sanya proposes that perhaps he is insane and this is a hallucination. Harry begins to laugh hysterically in response, probably mirroring the reader.
When Harry and Susan go to Michael's house, she comments on how his lawn is green in the middle of February. Harry's response?