The Gulf coast three hours from Bangkok that most foreign visitors skip entirely (Hua Hin, Cha-Am, Prachuap)
Most foreign tourists flying into Bangkok have the same plan: catch a flight or overnight bus and get to the islands as fast as possible. The whole western shore of the Gulf, three to four hours south by road, gets written off as the place Thai families go on long weekends. That's basically correct, and it's exactly why it's worth going.
Hua Hin is Thailand's first beach resort, the Thai royal family has kept a summer residence there since the 1920s, and the town has a comfortable, well-off, slightly staid character you won't find on the islands. No full moon parties. The crowd is families, retirees, golfers, and Bangkok couples down for the weekend. The night markets are good, the transport links are easy, and hotels run from cheap to genuinely nice. For most people it works as a base for the whole coast.
But the part I push people on is Prachuap Khiri Khan, another 90km south. It's a small provincial capital built around a curving bay with fishing boats offshore and a row of restaurants serving seafood that's about as fresh and cheap as you'll find anywhere in Thailand. Most menus are still in Thai. There is basically nothing there engineered for tourism, and it's exactly the version of Thailand most people say they're looking for when they book a trip to the islands.
In between the two, Khao Sam Roi Yot National Park has the single best day trip on this coast. A cave called Phraya Nakhon, reached by boat from a fishing village or a steep climb over a headland, has a royal pavilion inside it and a hole in the roof that drops a shaft of sunlight directly onto it at around 10am on a clear day. It's the kind of thing that sounds like a travel blog cliché until you're actually standing there.
Read more here: https://thai-travel-guide.com/blog/hua-hin-cha-am-prachuap-khiri-khan