never a wrong time to go to Thailand have been in almost every month of the year including over xmas and new year - plus songkran - kings birthday etc etc - but for what you are looking May sounds about right or possibly Oct / Nov rather than koh samui I would stick with phuket with hire car and drive round to krabi for a couple of days - also do BKK and chaing mai - or a good substitute could be around river kwai - where there is lots of cuture - sites - waterfalls - tiger temple etc and is reachable both by rail and car from BKK quite easily