Check out this place:
https://www.rayavadee.com/en/
It's not an island, but the only way to get there is by boat. Amazing scenery, great food. Very quiet at night, but there are some bars. Went for my wife's 40th years ago, amazing place. We had a rate that included breakfast, dinner and everything in the mini bar. Stay at a 5* hotel in Bangkok for a few days, then fly to Krabi to get there.