You're correct that the options are limited, due to the monsoon season which makes the East Coast of Malaysia a less attractive option (though on the plus side, it does make it attractively priced). Apart from Phuket, there's also direct flights from SIN and BKK to Krabi, which *might* throw up more reasonably priced options as well (though pretty much everywhere touristy around Phuket will raise their prices substantially between Christmas and early-January).

You could also go to Penang I guess, but it's not much of a beach destination (there are beaches there, but I wouldn't recommend swimming in the water). Koh Samui is accessible from both SIN and BKK as well, but flights aren't cheap and the weather's likely to be somewhat wet there too.

Wish you luck finding some peace away from the hordes there anyway!