My travel will begin on 28th Dec from Chiang Mai as I will have a annual meeting there. On 6th Jan I should depart from Bangkok back to China.First I hope to visit Chiang Mai and Chiang Rai if it is possible. Then go to Islands. I know there are many stunning islands in Thailand. I hope to choose the ones peaceful and prices there are not so expensive. I read a blog about travel in Trang,Koh Sukorn,Koh Kradan,Koh Libong and Koh Ngai. They are nice but seems too far from Chiang Mai. Is there anyone who can tell me the itinerary to get there. Hope it is not too expensive and not so safe. Or who can give me recommendation of other beautiful islands?
Thank you!

is a long way away from any islands, so maybe Samui? Because you can fly Chiang Mai - BKK - Samui. Anywhere else is going to mean a bit longer of a journey.

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As Markadmin said, Chiang Rai are northest province of country event Chiang Mai also made no different to get the nearest Islands. Samui is the first choice and Phuket and yes I would recommend Koh Chang as Bangkok Airways having flight to, just 40 minutes.