Thailand's planning question is not where to go, it is when — because the two coasts have opposite rainy seasons and the month decides which half of the country works.
The two coasts
| Coast | Islands | Wettest |
|---|---|---|
| Andaman (west) | Phuket, Krabi, Koh Lanta, Phi Phi | May–Oct |
| Gulf (east) | Koh Samui, Koh Phangan, Koh Tao | Oct–Dec |
Which means: travelling in June to September, go to the Gulf side. Travelling in November, go to the Andaman side. Travelling January to March, both are fine.
People book Phuket in July because it is high season at home, then spend a week indoors. The Gulf islands are two hours away and dry.
Burning season, which nobody mentions
Roughly February to April, agricultural burning fills the northern valleys with smoke and air quality in Chiang Mai can become genuinely poor — bad enough that people with respiratory conditions should avoid it.
It coincides with the dry season that looks ideal on paper. Check air quality forecasts before booking the north in spring.
How much time each region needs
Bangkok: 2–3 days. Temples, markets, food, the river. More than three and most people are ready to move.
The north: 4–7 days. Chiang Mai plus day trips, or add Chiang Rai and Pai. Cooler, greener, and considerably cheaper. It is the most commonly cut region and the most commonly regretted.
Islands: 5–7 days minimum. Because getting there eats a day at each end.
Isan, the northeast: almost no visitors, Khmer temples, the best food in the country. For a second trip.
Two weeks, realistically
Two regions. Bangkok plus the north, or Bangkok plus one island group.
Domestic flights are cheap — often €30–50 — which tempts people into three internal flights in fourteen days. The result is a lot of airports.
A working two-week shape: Bangkok 3, Chiang Mai 5, fly south, islands 5, fly home from Phuket or Krabi. One internal flight each way and no backtracking.
Getting around
Overnight trains Bangkok to Chiang Mai — second-class sleeper is comfortable and saves a hotel night.
Buses are extensive and cheap; the VIP overnight coaches are fine.
Ferries to the islands run to season and weather. Build in a buffer day before an international flight — cancellations happen and missing your flight home because of a ferry is a real risk in the wet season.
Practical
Songkran, mid April, is the water-throwing new year. Enormous fun and total chaos — transport is packed and much closes. Deliberate choice, not accidental timing.
Scooters. Most travel insurance requires the correct licence category and a helmet. Accidents on Thai islands are the single most common serious injury among visitors.
Temple dress code — shoulders and knees covered — and never point your feet at a Buddha image.
Bring your own water bottle; refill stations are common and bottled water waste on the islands is severe.