The last chapter of Harry Potter and the Chamber of Secrets ties up all the loose ends of the novel. Ginny is reunited safely with the Weasley family. Immensely annoying Professor Lockhart leaves his job at Hogwarts because his memory has been permanently damaged (thanks to Ron's faulty wand). Harry realizes that Lucius Malfoy is the one who slipped Riddle's diary to Ginny during the scuffle at Flourish and Blotts at the beginning of the novel. Harry uses the diary as a way to force Lucius to free Dobby, his house-elf. So Dobby is no longer tied to a Dark wizarding family. At the celebration feast the evening of all of this adventure, Hermione and all the other Petrified students return to school. All seems (mostly) right with the world.
The key point about the end of Chamber of Secrets is that Professor Dumbledore has a chance to emphasize the importance of choice in determining who you are. Both Harry and Tom Riddle share a lot of characteristics: a lonely childhood as an orphan wizard raised by Muggles, lots of talent, and even Parseltongue (the ability to talk to snakes). But Riddle responded to his miserable youth by becoming Lord Voldemort. Harry has chosen time and time again to stay loyal to his friends and his principles. Even though Voldemort has marked Harry with some of his own powers, Harry can still use those traditionally Slytherin talents for Gryffindor-ish purposes.
The thing that's odd about this conclusion is that, yes, Harry gets to choose to be in Gryffindor. He chooses to stand on the side of good, which makes him the boy hero we know and love. Later on in the books, there are all kinds of constraints on Harry's choices, what with Voldemort's manipulations and various prophecies. So the issue of free choice versus fate that seems so firmly decided by the end of Book 2 actually grows more ambiguous as the novels continue. Chamber of Secrets still presents a relatively black-and-white view of Good triumphing over Evil. This stark morality starts to fall apart as we get to Books 5 and onward, when the way forward for Harry seems less and less clear.
This sharply black-and-white depiction of morality is what makes Chamber of Secrets seem more like a kid's book than the later, harder books in the series. Once Voldemort rises at the end of Goblet of Fire, we get major multi-book plots involving the war effort against Voldemort, Snape's secrets, and the escaped Death Eaters.
By contrast, at the end of Book 2, we still know that Voldemort is somewhere out there and that there are Dark forces threatening Harry. Those forces, though, remain vague and unknown. Harry has won a decisive battle against them. He hasn't yet suffered the kinds of losses that mark Books 4, 5, and 6. That's one reason why Chamber of Secrets sometimes feels more pleasurable to read than, for example, Harry Potter and the Order of the Phoenix: Harry's decisions are easy and satisfying, and he wins everything in the end. The complexity of the later books makes them more mature and morally interesting, but they also become less total fun than Books 1 through 3.