I've been twice. Both in Dec.
First time was 2013.
Did the usual Bangkok for 3 days then the remaining 11 on Koh Samui and Koh Tao.
Didn't like Koh Samui. It's beautiful but full of the nasty side of Thailand.
Koh Tao though was great, small and quite island very romantic.
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Second time I went was in 2015.
This time I did the opposite to last time:
Bangkok just for one night. Once you've done Bangkok once you don't want to do it again imo.
Flew to Chiang Mai in the north. LOVED this place. Your in the mountains. Fresh air, national parks, elephant sanctuaries.
Stayed for about 5 days and didn't get bored. Its small but pre book activities.
Then took a 3 day boat to Luang Prabang in Lao. This was great as well. Your on the boat for a while each day (cant remember how long) but you make friends with the people on the boat and end up making friendships.
Spent 5 days I think in Luang Prabang. Possibly too long as there wasn't that much to do here. I'd say you could stay 3 days but not more.
From here I flew to Siem Reap in Cambodia to see Angkor Wat. I think I was here for 5 days as well. Spent 3 days seeing as much of the temples in Siem Reap as I could. Possibly 5 days is too long. May 3 again is enough.
From here I flew back to Thailand to do 1 week at the beach. As I wanted to be close to Bangkok for my flight back home I decided to settle on Krabi for the week. Not bad, but perhaps with 1 week to spare for a beach holiday there might of been better places to go.
All in all that took me 4 weeks and I loved it. Chaing Mai is a must imo.
Air Asia does cheap internal flights. So flight to Chiang Mai was cheap and quick.
Depending on how much time you have I would honestly try and squeeze in Chiang Mai.
If you've got two weeks I would stay Bangkok maybe 2 days leave on the third, fly to Chiang Mai for maybe 4 days leave on the 5th then you fly south and spend the last week at the beach.
To get to Koh Tao I think you have to take a few boats. Off memory you have to fly from Bangkok to another airport in the south and then take two boats.
There might be a direct boat, but when I went it was Koh Samui then Koh Tao.
This is the national park in
Chiang Mai.
But squeezing in Bangkok, Chiang Mai and a south island for beach you will basically have a nice contrast of what Thailand has to offer.
Busy and noisy Bangkok. Mountain fresh Chiang Mai and hot and sandy island in the south.
You might want to do some research on what island you want to go too. There maybe nicer places than Koh Tao. Its not that Koh Tao isnt nice it is, its just there might be nicer ones.
Phi Phi looks nice from pics.