When the news of Hiccup paying a literal King's ransom as the bride price for Astrid are told to outsidersnote (she is a love match from his home village, and her family is on excellent terms with the chief so the need to buy them off is not there), the reactions range from “This is not my first language so I must have missed something.” to “What happened to signing over a couple of cows?” and are always good for a chuckle.
Stoick's continuous insistence that he is a chief and not a king, in spite of the fact his holdings are kingdom-sized now. Other peoples' response are to either humour him and imply he's a king anyway (Rikard), not bother with any pretensions (Bladewit), or call him Chief to his face and refer to him as King when he's out of earshot (nearly everybody else).
Other
One of Trader Johann's sailors whistles at Astrid's appearance only to be interrupted by one of his fellows on the grounds of that catcalling is simply not safe when the girls in question are Vikings who ride dragons.
During the first Thawfest of the story, a mention is made on Hiccup's ability to hit a target with a thrown axe. He managed to hit the target! Once, in the five rounds. And that was with the handle.
In Chapter 18, Snotlout made it clear to Fritjof, who is his father's best friend, that the only way he can stop Snotlout from leaving to join the Varangian Guard is by knocking him out and strapping him to Nott and Delling's back. Fritjof then proceeds to try just that!
Hiccup's plan managed to defeat Harthacnut's navy! Great! The problem though was that it required materials, including Magnhild's pots, and she's upset about it. The only thing her staff could do was throw Hiccup under the bus and stay outside the danger zone.
When the Hooligans infiltrate Adalwin's (Alvin the Treacherous) stronghold, he sums it up quite simply as his guards are slaughtered and squads of dragon run amuck. "Well, this... is a problem."
Astrid's reaction to the teasings of her extended family as she learns her and Hiccup's "secret" premarital sex wasn't really a secret after all.
When speculating on what Hiccup's morning gift to Astrid will be, someone jokingly guesses a saddle that'll let Astrid ride Ratatoskr up and down Yggdrassil. After a brief pause, someone else notes that this isHiccup they're talking about...
Veisafjord becomes a Berkian territory in the most spectacular way possible. They conquered themselves and offered themselves to Hiccup, wanting the same prosperity Vedrafjord has.
In Chapter 51, after a few days of Hiccup upset that some of the freedmen of Vedrarfjord have started to look at him as a god, Astrid and Wulfhild decide to take matters into their own hands by hauling Hiccup back to their hut where, to quote Wulfhild, "We're going to make you make us sing your praises."
Hiccup: But I have a terrible singing voice! Astrid: That's all right. We have other uses for your mouth.
In Chapter 52, Thorred gives a hilarious rhyme:
Thorred: But if I don't set it to verse, in order to let Sigurd's tale disperse, and let others hear it and converse, things for Sigurd could get more adverse!
And then Gunnar responds in kind:
Gunnar: How much of that perverse rhyme did you rehearse?
Hiccup predicting that when Stoick hears that Astrid and Wulfhid are both pregnant, he'll run up Raven's Peak, blow the signal horn, and proclaim to the whole village, "I'm going to be a grandfather! Twice!" Sure enough...
Ruffnut tells Magnus and Yngvarr the origins of the Ingerman library. One of them wanted to marry Dror, and they decided to get lots of books as the dowry. Being vikings, they got it by raiding, which prompted Yngvarr to comment that it must have been one weird raid.
Yngvarr: Gold? No, not good. And what's with all this silver? Where do you keep the books?
Elinor catches Cami and Merida having just consummated their relationship and isn't happy at first.
Merida: Mum, I can explain!
Elinor: Explain!? You… you… Merida, do you realize what you've done!? And you! Is this why you were so friendly with her? So that you could seduce her to your ways?
Cami:: What 'ways'? No, I was tryin' to be a friend—at least at first!
Elinor: You're as naked as a wee babe. I'd say we're well past 'at first'!
Merida: Mum! You told me to make friends with her! 'She's close to the dragon-tribe'! Remember!?
Later, when Fergus is explaining to Stoick that Cami has effectively proposed to Merida, the latter can only burst out laughing at the idea of the former having Cami as an in-law, giving his congratulations and condolences.
In Chapter 71, Hiccup says Yngvarr is the only one they can have chair a religious synod, which prompts Gothi to thwack him. Hiccup defends himself by saying that due to her muteness, she can't shout down arguing priests, which prompts her to thwack him again.
In Chapter 72, dragons show they are capable of crude humour, with one Gronckle farting musically.
In Chapter 83, Sigurd's neighbours have a hilarious Mass "Oh, Crap!" reaction when he pays them a visit.
Also in Chapter 83, Gunnar helps himself to some cakes Sigurd's neighbors have set out during the above visit. When him and the other dragon riders leave, he asks for the recipe before saying they are delicious. The neighbors, no doubt terrified their home will be burnt to the ground by dragons if they refuse, agree to give it to him.
In Chapter 94, it turns out that the dragons' name for the Thunderdrum is... the Loudspeaker.
In Chapter 95, Gunnar makes a joke about the polycule that forms between him, Sigurd, Sophia, and Heidrun, which has three Norse and one Roman. He tells the sole Roman member, "Marry this Norseman and get two more".
In Chapter 98, Sophia makes a Call-Back to Gunnar's joke to justify her, Sigurd, Heidrun, and Gunnar having sex before her and Sigurd's wedding, saying, "I want to try before I buy."
In Chapter 106, Stormfly rolls on the ground doing the dragon equivalent of laughing when Hiccup mistakes a female Night Fury Toothless found as Toothless' new girlfriend, when in reality, it was Toothless' sister.
In Chapter 107, Toothless is indignant at the fact his sister gets a cool name like Fearless when he doesn't have a cool name.
In Chapter 118, The Battle of Inbhir Niss has left Alba without its King, like all the other lands that had attacked Berk in some way. Stoick is not happy, as it means there's another land he now he has to decide the fate of.
"What in Freyr's name am I going to do about Alba now?"
In Chapter 140, Mushu's insults against another dragon includes declaring dishonour on their fish. The other dragon has to take a moment to react to the absurdity of it.
In Chapter 146, Mushu thinks that the ship's crew locking up the food so he can't steal it is just a game they're playing with him, to see if he can figure out how to undo the locks.
In Chapter 147, Esther's dragon Keshet behaves exactly like a giant, scaly cat, begging for food.
Clearly he hadn’t ever been fed, no, no, not ever, and only another piece of challah would keep him from expiring right there out of hunger!
