A workable Thailand route groups places that share a transport corridor. Bangkok connects the country, but every return through the capital costs time; a shorter regional trip often produces more usable days than a longer list.
Bangkok pairs naturally with Ayutthaya and Kanchanaburi. Chiang Mai and the northern mountains form another cluster. Samui, Phangan and Tao share a Gulf island corridor, while Phuket and Krabi anchor the Andaman side.
Combining north and south can make sense on a longer trip with a flight between regions. Combining both coasts and the north in a short holiday usually replaces destination time with airports, piers and hotel changes.
Count a long bus, train, flight-plus-ferry or cross-island move as a transfer day until the timetable proves otherwise. Arrival time, terminal location and hotel check-in matter more than scheduled travel duration alone.
Sea and rain patterns differ by coast, and mountain conditions differ again. Use the Thai Meteorological Department forecast near departure and keep the most weather-sensitive day movable.