Harry makes some remarkably bad decisions over the course of The Chamber Of Secrets - starting when he and Ron decide to drive a flying car to school, rather than simply wait to let an adult know that they couldn't get through the barrier! Over the course of the year, Harry decides not to tell Dumbledore about hearing the voice in the walls, to write in a diary without knowing what it really was, to follow spiders into the Forbidden Forest...
Despite this slew of bad decisions, though, Harry does make some smart moves in The Chamber of Secrets. And as usual, for the Boy Who Lived, everything turns out well for him in the end.
Stopping The Snake
During the first, ill-fated Duelling Club, Harry accidentally reveals to the school that he is a Parseltongue. Draco produces a snake, which is about to go for another student - Justin Finch-Fletchley - but Harry uses Parseltongue to call off its attack. This has two positive effects. First, it saves Justin, who would otherwise have been attacked, if not killed, by the snake. More importantly (although likely not as far as Justin is concerned) this is how Hermione and Ron know that Harry can speak to snakes. It's not likely he would have mentioned it, otherwise, and it becomes a vital piece of the puzzle in figuring out that the Monster is a Basilisk.
Of course, there are some downsides to this decision, too. The students are suspicious of Harry being the heir himself, and this makes it much, much worse, because they couldn't understand what Harry was saying, and it sounded like he was egging the snake on!
Using Polyjuice Potion
The decision to get Hermione to brew Polyjuice Potion, and to use it to try and infiltrate Slytherin and trick Draco into revealing his secrets, was a hugely risky one - of course, the Golden Trio are known for taking big risks! However, this also paid off in a big way.
While Ron and Harry didn't get to learn what they thought they would, finding out that Draco was absolutely not the heir of Slytherin, and had nothing to do with the Chamber, was hugely important. This knowledge allowed them to focus their attentions elsewhere, finally figuring out the truth. There was a fairly big downside to this too, though, as Hermione learned firsthand what can go wrong if a cat hair is put into the potion by mistake!
Going To Lockhart
Asking Lockhart for help seems like it would be a terrible decision - given that he was a useless professor, and as fans know by the end, a liar who couldn't actually work defensive magic at all! However, this is one of the few times that Harry and Ron actually choose to make a calculated decision about how to proceed with a difficult situation, rather than just rushing into it.
For the most part, Harry and his friends tend to rush to the rescue without even considering alerting a more experienced adult about their knowledge - so this shows some maturity in the boys. In the end, too, while Lockhart was absolutely no help with the Chamber itself, the backfiring Memory Charm did the world a favor, as it put him out of commission, and prevented him wiping anyone else's memory in order to steal their stories for another book.
Defending Dumbledore To Tom
The major, life-saving decision that Harry made this term came in the Chamber of Secrets itself, when Tom was setting the Basilisk on Harry. Harry, despite his situation, decided to defend Dumbledore to Tom when Riddle was insulting the Headmaster - a choice that led to Fawkes flying into the Chamber with the Sorting Hat.
This, obviously, saved Harry in the end. The Sorting Hat provided him with the Sword of Gryffindor, which he used to kill the Basilisk - making its fangs available to destroy the diary and save Ginny. On top of that, Fawkes gave them the ability to fly out of the Chamber of Secrets, as it's not quite clear how they would get out, otherwise! All in all, the choice to defend Dumbledore saved the day, so it's no wonder that Harry continues to passionately defend his Headmaster for the rest of the series.
Giving Dobby A Sock
Harry's treatment of Dobby throughout The Chamber of Secrets was a great choice - and it made the House Elf a true ally to him. His decision at the end of the movie to find a way to force Lucius to free him, though, was absolutely inspired. Given that Dobby had spent much of the past year trying to 'help' him in ways that nearly got Harry killed, this was also a generous choice, and many young wizards wouldn't have thought to make it.
This was more than simply kind to Dobby, though. By making Dobby a devoted ally here, Harry's life is saved later, when he is imprisoned in Malfoy Manor, and Dobby comes to the rescue - even sacrificing his own life for his friend.