Thailand is the easiest long-haul trip in Asia to get right: a direct overnight flight, easy hotel service and food worth the journey on its own. The skill is choosing which Thailand you want: Bangkok, the northern temples or the islands.
The two coasts run on different weather. The Andaman side, with Phuket, Krabi and Phang Nga Bay, is best from November to April. The Gulf, with Koh Samui, stays drier through summer and takes its heaviest rain from October to December. We match the beach to your dates.
The honest caveat is crowds. Phi Phi fills with day boats by mid-morning and parts of Phuket are overbuilt, so we steer towards Phang Nga, Koh Yao Noi and the quieter bays of Samui, and time the famous sights for early starts.