Bangkok is the capital and where most trips start. It packs temples, mega-malls, Safari World, and river cruises into one city. Give it 2-3 days minimum.
Phuket is Thailand's biggest island, with Patong beach, tiger parks, and boat trips to the Phi Phi islands. It suits couples and families who want beach plus nightlife in one place.
Pattaya is 2 hours from Bangkok by road, which makes it the standard add-on to a Bangkok trip. Expect cabaret shows, water parks, and coral island tours. It's louder and cheaper than Phuket.
Krabi is quieter than Phuket with better scenery — limestone cliffs and the four-island boat tours from Ao Nang. Pick it over Phuket if you care more about beaches than nightlife.
Koh Samui and Chiang Mai suit second trips: Samui for a slower island holiday, Chiang Mai for temples, night markets, and elephant sanctuaries in the north. Samui is on the east coast, so its weather is at its best when Phuket's is at its worst.
Hua Hin is the quiet beach option 3 hours from Bangkok — water parks, night markets, and none of Pattaya's noise, which is exactly why Thai families pick it. Chiang Rai pairs with Chiang Mai for the White Temple, and Kanchanaburi is Bangkok's history day trip: the Bridge on the River Kwai and Erawan waterfalls.