We had 3 nights on Koh Tao and loved it. Thanks to all who offered advice especially DE lushylynds for the Koh Tao Resort recommendation which suited us perfect. Also MasterDivers for confirming current ferry timetables which took a lot of frustration / stress out of planning our itinerary. Koh Tao was everything we had hoped for. Could have spent more time there for sure. Just swam, read, walked and eat / drink at some great restaurants.
The Ying Yang at Chalok Bay was great for food. Try their Massaman curries.
Went on a day snorkelling trip with one of the commercial operators. Enjoyed very much, copped sunburn. Nangyuan island was a bit touristy though.
Would like to return some day and maybe ( against my better judgement ) hire a motorbike. I think it's the best way to really explore the island.