The first shot of the entire film is Helena playing out a scene with the Light Queen and the Dark Queen using black and white socks.
Helena (as the Dark Queen):Ah, you may think me a hard-hearted, black sock, but underneath this cold, dark exterior is a....naked, pink foot.
The Prime Minister, while looking for the Charm (before we know that it is the titular Mirror Mask) has a line of people with items they believe are said charm. This includes half a brick and a chicken.
Which moments later, when he mentions not knowing what it looks like, leads to this line:
Prime Minister: What if it WAS the chicken!?
Helena's riddle. What's green, hangs on a wall, and whistles? A herring. You can paint it green, you CAN hang it on a wall, and...well, she added the last part so it wouldn't be too obvious.
When distracting the Sphinx, Valentine first throw it off guard with a nonsensical riddle, then points and declares "Look! An idiot!"
Plus, the Sphinx's inability to understand that Valentine is actually telling him the answer to the riddle. "If you have it, you want to share it. If you share it, you haven't got it? It's a 'secret'!"
It's a little squicky (and a tad bit horrifying) but seeing the Dark Queen casually spit up some shadows over dinner to check on a loud rumbling noise is kind of funny.
This exchange between Helena and Valentine
Helena: Are you thinking what I'm thinking?
Valentine: Absolutely, if we put little wheels on the bottom of our shoes we could just rooooooll around...
That's the future fruit. Only eaten by holy people. It takes 900 years to grow and they write entire books about the things they have seen. And Valentine is the one who eats it.
Valentine: NOOOOOO! I DON'T WANT TO BE A WAITER!
At the very end, a real life counterpart of Valentine appears and talks to Helena, saying he's always wanted to join the circus. She tells him that he'd make a terrible waiter and they share a laugh. And then:
Valentine's counterpart: What?
If you're watching the scene at the mask shop again, focus on that music that Mrs. Bagwell is playing. It is just so hilariouslyweird; yes, the music is fine, but the singing at times is just random howling.
The full song can be listened to alone on the soundtrack; it includes a sudden growl of "SAUSAGES?!" and slightly creepy giggling of "You can't see me!" and "Silly boy! Where are you, silly boy?"