may is a great time to go IMHO its alot cheaper for accomadation and for flights its a little cooler than the asian summer although a little more humid - and not going to rain much - you dont have to queue as much as there are less people there etc etc - I would do phuket bkk and either river kwai area or chaing mai / rai - or you could try krabi - I am not a big pattaya lover but as others have said it depends on what you are looking for