After having all his written warnings ignored, Charles finally resorts to the following doodles on a blackboard: Raoul + chemicals = BOOM!
This exchange:
Lucille: I told you to stay quiet! What part of it didn't you understand, "stay" or "quiet"?! [Francœur strums his guitar awkwardly with a "sorry" expression]
"That's an extremus... gigantus... uh, circus... asparagus." "Actually, that's a sequoia."
Émile threatening to blackmail Raoul:
Émile: Give it back! I'LL TELL EVERYONE THAT YOU'RE IN LOVE WITH LUCILLE!
When Francœur plays the monster, he tries to roar... but sings instead. Made even funnier by his mortified expression during the scene.
Maynott and Paté accidentally breathe helium and have squeaky voices. UPROARIOUS!!
And this line shortly after Maynott gets affected by the helium:
Maynott: The monster stands between me and the city hall! He dies... [laughs crazily] and I win! [continues laughing crazily]
And a bit earlier, while Paté is pedaling on a bike:
Maynott: Faster, Paté! Imagine you're doing the Tour de France! Paté: I never intended to! Too hard! Maynott: Well don't complain, we could be on a mountain road!
Following this, "Ooh, I'm so dumb! Oh, I fell on my butt again!"
The whole argument scene, really, especially when Lucille growls at Raoul when he calls her "honey".
When it's revealed that Lucille and Raoul met when they were kids. His toy car rolled off and hit the back of her shoe, she picked it up, he smiled sheepishly at her... and then Lucille stuck her tongue out at him and ran behind a tree with it. Raoul burst into tears and ran off.
Even better, during their kiss when everything's cleared up, Raoul stops to ask Lucille if she still has the toy.
When asked if they've ever heard such beautiful singing before, Raoul and Émile briefly flashback to Charles singing, before replying with a yes.