changmai is great, woukd stay there a few days, also check out best breakfast place there on tripadvisor, had the bwst breakfast in my life there😃.
Phuket is nice but I would go to Krabi if you want to see nice beaches/ islands. I believe phi phi is being renovated double check. Rallye beach is nice. Phuket is just a lot of markets and sex industry.... banglaroad and kangaroo bar are a must see.
We enjoyef koh samui as well!
Bangkok my tip is use taxi in stead of tuk tuk just as cheap, the air in Bangkok you are better off not breathing!
Thanks for the tips.