Thailand is really one of my favourite holiday destinations. People are very friendly, the food is excellent and healthy, the country features splendid beaches as well as a rich cultural heritage, and the means of transports and communication are well developed. Plus the warm climate which really makes you want to travel there all the times.
My personal recommendation is to start with Bangkok (กรุงเทพมหานคร) and explore the sightseeings there as well as the excellent cuisine. Then either start off to the South of the country (where you also have beautiful islands) and its marvellous beaches with a stop-over in Hua Hin (หัวหิน) or to the much quieter location Sam Roi Yot (สามร้อยยอด), or start off to the East and spend some weeks on Koh Chang (เกาะช้าง), an island which is not overflown by toursists, or you start off taking the long-distance bus to the cities in the North of Bangkok (กรุงเทพมหานคร), like Ayutthaya (พระนครศรีอยุธยา), Sukhothai (สุโขทัย), Chiang Mai (เชียงใหม่), Chiang Rai (เชียงราย), Nakhon Ratchasima (นครราชสีมา), Udon Thani (อุดรธานี), Ubon Ratchathani (อุบลราชธานี). There is so much to be seen, even in smaller villages.
Keep in mind that even places which have many tourists [like Koh Samui (เกาะสมุย)], you still encounter (contrary to what you might expect) excellent restaurants at reasonable cost.