Ena: (makes a terrified expression while watching Yotsuba fly off... who thankfully lands safely in the sandpit)
Fuuka's first encounter with Yotsuba... is when the latter is climbing up an utility pole, pretending to be a cicada. She then exclaims, "It's a weirdo!"
The Finnish translation tops it off. Yotsuba says "Olen kaskas" ("I'm a cicada"), which isn't funny yet. The thing is, "kas kas" is a common way to say "is that so?", which is exactly what Fuka says next, making this an epicly lame pun. It's like Kiyohiko Azuma wrote the line expecting the eventual Finnish translation.
Fuuka getting stuck in the Koiwais' bathroom window... not long after Koiwai did the exact same thing.
And then being made fun of by Asagi, who also brings out Ena just to have a good laugh.
Yotsuba playing with the tape measure, more specifically when she asks a distracted Fuuka to hold it while she gets enough distance and sends the tape flying. Dad was not happy.
"After that we went cicada hunting in Ena's house."
Chapter 7 when Koiwai randomly put the underwear on his head. That and everything else involving "Pants Man!!" (A.K.A. "Boxer Man"), including Pants Man's later nemesis, Bare-Butt Man - Yotsuba's sense of humor is, well, so five-years-old.
Chapter 9 has Yotsuba, after watching a gangster movie, going on a revenge rampage on her father, Jumbo and the neighbors with her water pistol. From squirting down her own father in cold blood, to taking out one of Ena's teddy bears (while saying she doesn't kill chidren), to the part where Asagi gets the drop on her and gives her a taste of her own medicine.
Chapter 12, "Yotsuba and the Pool":
Fuuka pushes Jumbo in the pool and makes fun of him for not being able to swim. Koiwai defends him, and so Fuuka also pushes him too and finds out he also can't swim. She laughs at the two of them... and then Yotsuba also pushes her in, and it's shown she also can't swim.
Koiwai: So, you can't swim either.
Fuuka: Yes. Sorry.
Yotsuba as a swimming instructor
Yotsuba: Let's start with pike.
Koiwai, Jumbo, Fuuka: ........
Yotsuba: You have to think about the fish!
Caption: Yotsuba's swimming dojo - unsuccessful.
Chapter 13 has Yotsuba stalking her prey, Fuuka's reaction to the frog, Koiwai's visitor, and the Reactive Continuous Scream from Miura, Ena and Yotsuba at the end.
Asagi's CDs for her father in chapter 14.
Asagi: Hm... I wasn't thinking when I bought these. Dad... isn't... with us anymore, is he?
[Asagi and Mrs. Ayase look out the window as if mourning his death]
Fuuka: No, no, no. Dad is alive. He's working right now.
Ena: Asagi always likes to kill Dad off.
Koiwai dressing Yotsuba up as the Flower Cupid.
Koiwai: Very cute! Just like the Little Match Girl. Yotsuba: Ahh, the one who sets houses on fire? Koiwai: No, not at all. Which one is that?
Chapter 23 has Jumbo going berserk when he finds out Miura is going to Hawaii.
Jumbo: And now our little princess will perform a hula dance!
Ena:<clap> <clap> <clap>
Miura: STOP THAT!
Yotsuba attempts to cure Fuuka's heartbreak, after Fuuka has seen her crush hand in hand with another girl... unfortunately Yotsuba's advice is under the influence of Asagi, and so includes such gems as "Asagi's never known heartbreak!", "You have fat legs!" and "Your thighs are pudgy, too!".
When Fuuka tries to explain heartbreak to Yotsuba. She asks her who she likes, and she replies with, "Daddy!" Fuuka asks her to imagine her dad not liking her anymore... Yotsuba takes it too seriously and really tears up at the thought of it, and Fuuka quickly reassures her her dad loves her.
In Chapter 27, Yotsuba finding out from Asagi and revealing to Dad what tsukutsukuboshi really are.
Yanda's introduction, beginning an instant rivalry with Yotsuba. He bites her ice cream, she trolls his phone conversation, and then he prank calls her. Basically, their first meeting introduces the funniest rivalry ever.
A little later, they take a break and Jumbo gets hamburgers. In an adorable, and hilarious, moment, Yotsuba maliciously eats hers at Yanda, who only had cup ramen.
In Chapter 46, when Yotsuba is shopping for ramen, she says, "With this much, I figure even if I ate it once a week, I could still have it every day." Another shopper has a hilarious look on her face as she tries to comprehend that sentence.
In Chapter 47, the faces Yanda makes when Yotsuba is trying to get a ticket from a toll booth (and poor Yotsuba's subsequent reactions).
Chapter 48, the farm chapter:
Yotsuba trips and falls down the hill, smashing her head on a sheep. Both Yanda and Jumbo laugh at her, but Yotsuba just does a grumpy victory sign to Yanda. And then she apologises to the sheep, it attacks her with its head, and then Yotsuba punches it in the face.
When Yotusba is going to punch a cow to prove to Yanda that she's not a coward.
When the gang are being taught by the tour guide how to milk the cow:
Pretty much the entirety of Chapter 69 pt 2, but special mention goes to Miura's little attempt at wisdom on the first page:
Miura: Robot or not, all living things must die
Miura: Even though he's not a living thing...Since he's a robot...
Volume 11 adds a bonus section to chapter 72, featuring Yotsuba, Dad, and Jumbo hiding when Yanda comes round. Badly. And then Yanda hides himself too.
There's also this:
Yotsuba: But Daddy ate so much that he turned into Full Belly Man.
Jumbo: What does he do when he's Full Belly Man?
Yotsuba: He doesn't move.
Chapter 76 is melancholy, as Yotsuba is lamenting over Juralumin no longer speaking. Fuuka decides she'll pretend to be Juralumin by proxy as a way to cheer her up. Yotsuba's "what the heck is your deal?" look is priceless.