Koh Lipe, Koh Lanta, Koh Phang Ngan, Koh Samui, Koh Chang, etc.
When we go we usually stay in a 5 star hotel directly on the beach so I enjoy it. (Phuket). You'll need 10,000-15,000 baht a day for a family a family of 5.
.... Or you could pretend you're on Holiday and stay in some dumpy hotel blocks from the beach eating street food. Still need 5,000 baht a day.