Far East can be dodgy weather wise at summertime but I have been there quite a few times and have had good weather in Ko Samui and Hua Hin in Thailand.
I think a 2 centre would be great , perhaps a week in Hua Hin , beaches there are not great but it's fab for shopping and there are some great hotels with lovely pool areas, and you can visit the floating market and bridge over the river kwai from there also go elephant trekking etc, I would follow this with 1 week in Ko Samui for the real paradise tropical island beaches, perhaps staying in Chong mon or Chaweng Noi as they are lovely areas with no seedy girly bars etc...
Hope this helps, I'm sure many others on this forum will have more good ideas , love shirleyV