Originally i was looking at going around August time but i believe that is right in the middle of rainy season. So probably May, June, July,

Ive not done a lot of research and plan to keep it that way as i want it to be a pretty spontaneous trip. But Bangkok, Chaing Mai, Phuket and Ko Samui i hear are the best places to go.

My Criteria is:
Culture
Beaches and relaxation (a hut on a beach)
Partying