Ziay!

SleepyJeanne

Is late September or early October a good time to visit?

crocodilexp

September or early October are not particularly good -- it's the middle of a rainy season, and it does flood sometimes. If that's the only time you can go, though, you can still have fun -- the rain in the tropics is only a minor annoyance, especially if you're an adventurous / backpacker type (if you're a granny on a package tour, then getting wet might completely ruin your vacation, oh, dear!).

Not sure for how long you're staying, but I think you should spend at least a few days in Chaing Mai (wonderful handicrafts, trekking, hilly countryside and beautiful temples), and some time on the beach (Koh Samet is my favorite, and reasonably close to Bangkok).

Bangkok is an amazing, vibrant city, and I don't think you'd be bored staying just in the city for a few months -- but if you're a tourist trying to relax on a vacation, it can be a bit too crowded and overwhelming.

Not trying to duplicate the Lonely Planet here -- just give a few suggestions

Iwae

I visited Ko Samui island during my stay. I really loved this island. The hotels are very cheap, the food is wonderful and the people are friendly. One negative thing is the number of foreign visitors there, but it definitley has a groovy "party" vibe about it. Also there is a "Full Moon Prty" held on the next island. FUN! Can't wait to get back.