Thailand sure is good but I didnt think much of the snorkelling (unless you go to ko tao) September is fine, i was there august, september and october. When its wet season in the west (phi phi, krabi, lanta) its dry on the east (phangan, samui, tao)
Why has no one said Fiji?
I did 1 night on the main island then 13 nights awesome adventures trip then 1 more night on the main land. The trip was 5 islands and one night on a water taxi. The cost of the awesome adventures trip was £350 which included all meals and snacks and extra activities including a full day on a yacht with all inclusive beers, wine and food, sunset/sunrise guided hikes, fishing trip with the locals, a cave trip and a few other things. I did stay in dorm rooms but for a few quid more you could stay in your own room.
Buy alcohol at the airport or take some with you. I took a litre of rum and a litre of vodka, which i expected to last a week. (It ended up lasting 4 days!)
Fiji is a truely amazing place as are the people, snorkelling was awesome (i went out by myself and saw sharks, stingray, loads of fish and other things) Im not sure how much flights are because i got mine from Aukland to Nadi. Im guessing around £500 or £600 from UK
just before sunrise
castaway island where i snorkelled then went on
view from sunset hike
where we stayed (the big dorm bottom middle)