It's often overlooked, but there are 500km of gulfside coastline between Bangkok and Surat Thani (opposite Samui). This section concentrates on the northerly 300km, from the capital to the self-appointed 'gateway to the south', Chumphon. Within this stretch, the northern half (Phetchaburi - Cha Am - Hua Hin) has succumbed to rather tasteless high-rise development, while the southern half (Prachuap Khiri Khan - Chumphon) is almost untouched by tourism. Spend your days discovering secluded beaches, snorkelling and diving, trekking in the national park and taking trips in longtail boats.
Chumphon has boat links with Koh Tao - Koh Phangang - Koh Samui, but timings might necessitate an overnight en route.