I'd say if you want to see the real Thailand only, avoid party islands and party cities. They're kind of designed places to attract tourism income, and you can't find genuine stuff there. There're hundreds of untouched islands in Thailand. I'd recommend Krabi, Ko Lanta, Khao Sok, Ko Ngai, Phetchaburi, Ayutthaya and Chiang Mai; it's a list of nice natural and historical places. Especially if you also avoid the crowded touristic season, you'll find much greater serenity and cheaper prices :)