King Caspian re-conquers the Lone Islands by sheer force of awesome because he doesn't have nearly enough men to take them by force of arms. When Governor Gumpas fails to display the proper degree of respect, Caspian impresses the seriousness of the situation upon him by overturning his table, summarily deposing him, and naming a more loyal nobleman Duke of the Lone Islands right in front of Gumpas' face.
The newly-appointed Duke Bern delivers the final You Suck to Gumpas.
"The question is whether you and the rest of your rabble will leave without a flogging or with one. You may choose which you prefer."
Reepicheep, a walking CMOA, gets a whole new one when, being swung around by his tail by a prankster four times his size, he has no trouble in drawing his sword and beating said prankster into a blubbering mess.
And when he has to be specifically forbidden from challenging a dragon to single combat. And his response to an island of the worst things imaginable, that is, basically calling everybody else wusses for wanting to leave. Frankly, he's pretty much awesome in convenient mouse form.
Caspian: "You may say what you like, Reepicheep. There are some things no man can face!" Reepicheep: "It is my very good fortune, then, not to be a man."
Reepicheep meets Eustace:
Eustace: "I can't stand trained animals! They're so pathetic!"
Reepicheep: (beat) "Am I to understand that this singularly discourteous person is under your majesties' protection? Because if not..."
The whole scene with the sea serpent.
Lucy going into the magician's house all alone to take the spell off the Dufflepuds.
From the Movie
In the movie Eustace, now a dragon, finally proves himself useful when he starts towing the Dawn Treader by his tail during a doldrum. This especially goes for the cheer from the crew who have reason to respect him at last.
Eustace pretty much churns them out throughout their mission to rescue the last Telmarine lord. First, he gets the lord off of the island and onto the ship, saving a lot of time and effort. Then, he attacks the sea serpent and does a fair job holding it off. Finally, when he's turned back into a human, the first thing he does is take the sword to Aslan's table, so the island can be destroyed and everyone will be saved. This is even more impressive when you consider how useless he was before.
In the fight with the sea serpent we finally get to see Lucy fight as she gets a perfect shot with an arrow into the creature's head.
In the movie, when Eustace is turned back from being a dragon, he grabs the final sword and runs all the way back up to Aslan's table to place it there and defeat the Dark Island's evil.
The series has a real life one just for Voyage of the Dawn Treader being made with almost all the relevant original cast members, after everyone thought the series was dead after the subpar box office of Prince Caspian.