Why go
The map explains why Chiang Mai, Bangkok, Isan and Phuket offer very different journeys. Doi Inthanon rises in the northern mountains; the Chao Phraya crosses the central plain; the Mekong borders the northeast. Farther south, the peninsula separates Gulf and Andaman coastal choices.
The full story
The north is a highland region, with Doi Inthanon standing above the surrounding ranges. Central Thailand has a flatter shape: the Chao Phraya valley runs toward the Gulf of Thailand and gives Bangkok its river setting. To the northeast, Isan occupies the Khorat Plateau, with the Mekong marking its eastern edge. These are useful distinctions when deciding whether to spend a trip among mountain scenery, inland towns or river landscapes. Southern Thailand narrows through the Kra Isthmus and continues down the Malay Peninsula. Its two sides face different seas. Phuket and Krabi sit on the Andaman side; the Gulf faces the opposite coast. A route that crosses from one region to another changes landscape as well as distance, so allow those regional shifts to shape where you base yourself.