Thailand is usually planned as two halves: Bangkok and the cultural north around Chiang Mai, and the beach destinations south of them — Phuket, Krabi, Koh Samui — plus Pattaya within reach of the capital. Domestic flights make combining them straightforward, and most trips do.
The guides here cover Bangkok, Pattaya, Krabi, Koh Samui and other destinations. Monsoon timing is the detail that catches people out: the two coasts have opposite rainy seasons, so the Andaman side and the Gulf side are rarely at their best in the same month. November through February is the driest and busiest stretch overall. Temples enforce dress codes covering shoulders and knees, which is worth knowing before a day built around them.