I have not been to Phi Phi, but we went straight from Khao Lak (after the Oktavia) to Koh Lanta and really enjoyed our time. It was pretty chilled family time for us. We stayed at
Sai Naam Lanta and the beach was nice down there and there were plenty of nice little restaurants on the main street. I only dived Koh Haa which was a nice simple little dive. I had some of the best viz I've ever had underwater anywhere the day I was there. I dived with Scubafish who were OK, but nothing particularly special.