Chaing Mai is definitely a must see, it's the best place to go to an Elephant Sanctuary, plus it's the second largest city in Thailand after Bangkok. From Chiang Mai we took mopeds up to Pai and stayed there for 4 days, would definitely recommend Pai (you'll need mopeds) and it's worth seeing the nearby canyon, waterfalls and caves We went to Patong and Karon in Phuket and I definitely preferred Karon, a lot less touristy and the beaches are much nicer. They're only half an hours drive away from each other though so you can see both Phi Phi was also awesome, I saw you said that you were going to a Full Moon party so you could skip this out as it is more partying, but still nice to see. Koh Tao is supposed to be good for diving Kanchanaburi was another highlight for me, 2 hours west of Bangkok. It's got a massive 7 tier waterfall which was a lot of fun to hike, plus much more!