Thailand forum

If you look at travelfish's great interactive weather site you will see that Ko Chang tends to be one of the wetter areas in August, Sept and October - although the wet season does tend to break there a bit earlier, and one resident expat reckons late Oct is frequently good.
I've not visited at these times, but I've seen plenty of posts saying they have had a good holiday on Chang. I think you will find that a fair few places are still open on White Sand beach, which tends to be OVER busy high season, and there will still be the chance for interaction with other travellers.

The southern Gulf islands of Samui/Phangan/Tao are usually much drier for your first 2 months, that area's wettest time is normally Oct into Jan. Phangan and Tao are normally considered the better bet for non-upmarket travellers - maybe you could check the pages in the link below plus of course travelfish's descriptions to see which areas appeal to you. Note that August is a kind of second high season after Xmas/New Year for these places.

The Andaman side is normally wetter but not as wet as big Chang (except up around Ranong). Places with a little life include Phi Phi, the Krabi area, Phuket itself (kinda a bit like Samui) and the Long Beach area of Ko Lanta.

Back towards big Chang, Ko Samet is a nice island which tends to be drier than most. Because of this it is never dull or deserted - actually it can be a bit hard to get a place walk-in on Fridays and Saturdays because of weekending Thais from Bankok and Rayong.