Koh Samui is a busy tropical island, December unfortunately weatherwise is not such a good time, there are regular flights from bangkok. if you want a quiet natural beach area within a couple of hours cab or limo ride from bangkok the south beach area and takiab bay in Hua hin are a very good option, beaches are not brilliant but weather is good and especialy takiab bay has a wide and long beach, nice for jogging, lazing, also there are Elephants there for ridesd and also wandering around, horse and buggy rides around local temples, horses, cycling, fishing and nearby golf. Khao takiab beach has luxury places like the Hyatt Regegcny, Wora Bura and Chiva Som plus botuique resorts, bungalows and guesthouses. Much quieter than Hua hin town center but only about 7 minutes from the very central market in Hua Hin to these quieter and nicer beaches, also local transport costs cents. If you want luxury go for Hyatt or Wora Bura, for boutique spa style Kaban Tamor / S'Mor Spa, if you like boutique B&B style then the Ocean Breeze "le club" at Takiab Bay in Hua Hin is excellent
( has links also for the main lodgings in the area )
In the other direction from Bangkok heading south east as opposed to south, Sunset Village just past Jomtien is relaxing.