AXON Award the Conrad Rangali. Whalesharks being spotted regularly. I recommend going there and diving with Whalesharks, and Dining in the Under Sea Restaurant. Other activities might include walking off your over water bungalow into the sea for a quick snorkel.
Phuket and Samui are rather naff compared to this, not a place I would pick to go with the wife again. Activities here include options such as a ping pong ball show and some rather tasty Pad Thai. Ko Tao is much nicer if it is to be Thailand.