Thanks for the advice. I should have specified in the OP we are flying in/out of KUL (thanks to an excellent flight deal) and looking forward to spending some time in Malaysia. I was attracted to Samui because it's only a short 1 hr flight from KUL, so it's an easy add on to the trip whereas Maldives is a bit of a hike. Da Nang looks not too far though so I will investigate there.
From KUL, Bali is a nice option too, and there you have a plethora of options. Koh Samui really isn't that good for what you're looking for.