Go to Koh Lanta, you can get the ferry from Phuket, takes 4 hours leaving at 0800 or 1300, goes via Koh Phi PHi (where the film The Beach was filmed). Koh Lanta had a great mix of “stuff” to do versus relaxing, gorgoues place. CAn you scuba dive? I’m guessing not but we learnt to dive before (finishing with OW dives whilst in Koh Lanta, can’t recommend Dive and Relax enough) and it was phenomenal. We are now addicted and have the bug.
Chiang Mai is also fantastic, but the complete opposite end of the country, mind you could get flights from Phuket to Chiang Mai for £30 odd and it is brilliant. you can definately MTB up there and there would LOADS of companies when I googled it, we didn’t go as the reason for being in Thailand was Mr MC had ruptured his tricep tendon earlier in the year so it was our first non MTB holiday in 8 years! They do 3 day treks which are brilliant only about £33 for food, guide, accommodation (basic shack with a tribe) but had amazing views, gorgeous food and was a real experience.
Just found this
http://singletrackworld.com/forum/topic/thailand-and-diving
not sure if anything ni that thread helps, it was started 9 months ago!