I would suggest you visit the non-touristy, smaller islands, such as phi phi, aur Koh Samed. You can book yourself an Airbnb (they're very beautiful and comfortable in Koh Samed), that way you could economise on your expenses, and also enjoy yourself. Spend a few days on the island and spend the rest of your days in Bangkok. the street food there is amazing! visit the night market, and food market. lovely place!