If you want quiet and relaxing I would strongly urge you to consider Koh Lanta. It is a bit more time consuming to get to than Phuket as you have to fly to Phuket or Krabi then take a ferry to Phi Phi then to lanta (about 1.5 hours per ferry). Phi Phi is fun but full of young tourists that want to party though I hear the north side of the island is quieter.