Why did Klous gave Cerolina a bracelet for her birthday and why was he biing so nice to her in the last episode (season3 episode 11)

i think hes tryin to get everyone on his side because they already dont like stefan...at least anymore maybe he wants them to turn against him

Difficult question. My best guess would be that he's working several angles. He just suffered a large loss with Stefan and now he may be trying to get one up on Stefan by attempting to get the sheriff to "support" him (which he will need if Klaus is the one whom killed Brain Walters). Also, did you see the look on Klaus's face as he entered the house and passed Matt? Playing on Klaus's lips was the slightest hint of a devious, conniving smile.
As to why he was so nice Caroline. I don't have the slightest clue. That part of the episode left me very confused. Perhaps Klaus does have feelings for Caroline or perhaps he was simply trying to get in her good graces or plant the seeds of doubt concerning the nature of his true character (is he really evil?).
No concrete opinions though. Maybe we'll find out in the next episode. Can't wait!

Well, we can't know his intentions yet, but it's been said he was going to have a love interest and it would be someone we know. Guess it's Caroline.

Because Caroline had been bite by Tyler and she was crying as she was going to die on her birthday and Klaus felt sorry for her.

That's the big question. No one REALLY knows yet.

Either he is trying to get her on side, so he won't have a problem with Tyler doing his bidding or he will try to date her, so that Tyler won't care about her no more, hence an angry hybrid, who don't give a s***. I think Klaus might start off by using her, but after that who knows.

he is a sucker for birthdays :)

He's in luv with her