As a girl, I have to vote for Costa Lanta on Ko Lanta. It's one of the few areas I have yet to visit
As a man, I make fun of you because you suggest a place you don't even know.
But ok it's a pretty nice place.
There is no nightlife on koh Lanta and basically nothing to do ( stay 3 days max I'd say ).
Koh Chang is probably my favorite island, quite beautiful and you can stay there a week without getting bored to death.
If the girl is worth it, the four seasons chiang mai is perfect IMO.