@Shahid **********
"Cold" is relative. Depending where you are, it does not go much below 15°C. Sometimes in the morning it does go lower a bit. Y'r question: Islands like e.g. Koh samui etc. are nice, kanchanaburi also for some history of the building of the famous 'bridge over the river Kwai' and maybe, if you want the Thai Isan feeling a bit, first go to Khao Yai national park and then visit also the town Nakhon Ratchasima, with some area and villages around it. If you not like it, just go to another part.