For 2 nights I would go to Hua Hin. Phuket requires longer time IMO as it will take you at least half a day or more to transfer to/from BKK (mind you the distance from HKT airport to Kamala, Patong, Karon, Kata etc is quite far; around 1 hour by car). Also everything is more expensive in Phuket than elsewhere in Thailand.
You could also consider Kanchanaburi as an alternative to the beach destination Hua Hin