Thailand forum

Try the Krabi Area; especially Ko Jum.
But do expect at least some rain.
In september there is usually not much rain on the mainland between Petchaburi and Chumphon.
No islands but the beaches are very quiet if you look around a bit.
My best experiences in 09/2006 and 09/2007 have been with this part of coastline; further down south the westcoast can be pretty rainy.
Also it helps to move around; if it rains in one place; 30km away the weather can be ok.

Normally the southern Gulf islands of Phangan, Tao and Samui tend to be drier in September - their wet season tends to run Oct into Jan. See TRAVELFISH'S excellent interractive weather map or the wet weather page in the link below.
A relaxing, not too touristy beach - Thong Nai Pan, Mae Hat, Hat Sadet on Phangan.

Other Thailand places are normally okay for travel with enough sunshine between one or a few storms/showers, prolonged wet periods not too common, several days with no rain at all. Wettest islands tend to be the Rangong and Trat area islands. Driest away from the southern Gulf - Ko Samet.
Relaxing, not too touristy beach - Ao Tubtim/Pudsa on Samet, Ao Toh Koh on Phi PHi, several on Jum, main beach on Ngai are a few of several.