For that time of year I'd favor the Gulf side rather than the Andaman side. On the gulf side you've got Ko Samui, Ko Pha-Ngan and Ko Tao, plus the Ang Thong Marine park. Also Ko Samet, Ko Chang, etc., on the eastern side (Samet being driest). Ko Samui is easily most-developed and maybe too much so for many people, though it does mean 4* and 5* places available.

I also liked the Perhentians in Malaysia a lot, though they aren't built up like the Thai places are.

And I have nothing against Krabi, but ventures to the Andaman side in monsoon season have shown a heavier monsoon than you get in other parts of the country, so I'd want a pretty deep discount before considering going then. Also, if you plan includes boat rides (such as a day over at Ko Phi-Phi) those can be put at risk by choppy waters.

With airfares I have much more trouble booking awards from the U.S. for the summer trip than for the winter one, even though winter is the high season for Thailand. The trans-Pacific segments are what mess things up.