
Chiang Mai is the antidote to Bangkok. Slower, cooler, and surrounded by mountains and jungle, it's the cultural heart of northern Thailand. The old city is a maze of temples, the night markets are some of the best in the country, and the cooking classes are world-class.
The old city is walkable. For further distances:

Chiang Mai is the best place in Thailand to do a Thai cooking class. Most include a market visit and cover 4–5 dishes. A half-day class costs around 1,000–1,500 THB and is one of the best value experiences in the country.

The hills around Chiang Mai are excellent for trekking. Most guesthouses can arrange 1–3 day treks into the jungle with hill tribe village visits. Go in November to February for the best weather. Avoid March to May when smoke from field burning fills the valley.
Give Chiang Mai at least four days. Most people wish they'd stayed longer.

Bangkok
Temples, street food, and relentless energy

Chiang Rai
The white temple, the blue temple and the black house

Pai
762 bends into the hills, and nobody leaves on time

Khao Sok
Floating bungalows under limestone, in rainforest older than the Amazon

Koh Tao
Where half the world learns to scuba dive