Regions to know
To frame your trip by what you have time for and what you're after.
Bangkok & Central
Bangkok, Ayutthaya, Kanchanaburi, Khao Yai
Bangkok as gateway — most flights land at Suvarnabhumi (BKK). Day trips: Ayutthaya (UNESCO ruins), Kanchanaburi (Bridge over the River Kwai + Erawan Falls), Khao Yai national park (oldest in Thailand, monkeys + waterfalls).
The North
Chiang Mai, Chiang Rai, Pai, Mae Hong Son
Cooler, mountainous, temple-dense. Chiang Mai as base, then Chiang Rai (White Temple, Blue Temple), Pai (hippie town in a valley), Mae Hong Son (remote loop with elephant sanctuaries — go to ethical ones only). Best November–February.
South Andaman
Phuket, Krabi, Phi Phi, Ko Lanta, Khao Lak
The famous coast — limestone karsts, turquoise water, the postcard Thailand. Best November–April; rough seas and closed boats May–October. Skip Patong (Phuket's tourist strip) for Krabi or Ko Lanta unless you want the party.
South Gulf
Ko Samui, Ko Phangan, Ko Tao
Opposite monsoon timing to the Andaman — best January–August. Ko Tao for cheap diving, Ko Phangan for the Full Moon Party (and a quieter west coast), Ko Samui for the comfort tier with direct international flights.
Isaan (Northeast)
Khon Kaen, Ubon Ratchathani, Khmer ruins, rural Thailand
The Thailand no one visits. Khmer ruins (Phimai, Phanom Rung — Angkor's cousins), spicy Isaan food (som tam, larb, sticky rice), almost no tourists. Add 3–5 days to a longer trip; not a fit for the first visit.
Western & Khao Sok
Khao Sok, Cheow Lan Lake, Erawan Falls
The wildest jungle south of Bangkok. Khao Sok's floating raft bungalows on Cheow Lan Lake are a bucket-list overnight. Erawan Falls (7-tier turquoise pools) is in Kanchanaburi province — easy day or overnight from Bangkok.