So, I started this post and had some wonderful advice, and we did end up going to Thailand mid July so have not been back long. Our first time in Thailand and we LOVED it!
Due to time limitations, and what I've heard about the weather, we did not do Chang Mai but we spent 4 nights in Bangkok followed by 7 nights in Koh Samui (50 min internal flight). For us, this was the perfect amount of time in each place (could have stayed a few more days in Samui, mind).
We chose a night flight to Bangkok which worked perfectly as we arrived local time mid/late PM, so we just chilled at our hotel that PM/eve (we opted for a riverside hotel - much calmer and some amazing choices, including with pools and roof top bars). Followed by a good night's sleep, we did not suffer from jetlag - result!
We then did a Bangkok local floating market (not the big one) tour with a local guide who was brilliant. It was not much of a floating market in terms of number of boats but absolutely amazing market and I only saw one other lot of tourists. That was the morning, we then did some shopping and chilling at roof top bar/restaurant in the evening.
After that we had one day where we did the usual temples and we were amazed at how few tourists there were (even though it was the weekend) - so no queues going into the main ones, same with the Grand Palace. Also fitted in China Town / Yaowarat Road which was great. We had been wanting the Bangkok hustle & bustle including street food.
We invested in a private tour (a/c car and local guide) to take us to Ayutthaya which was a little over an hour away for a whole-day trip. That was absolutely amazing, would really recommend that as the temple site was beautiful and, again, not busy at all. The guide also took us around to see how a local 'sweet' was made and took us to a local, inexpensive authentic restaurant (no Westerners as far as I could see) which was fab.
We used taxis, ferries and, once, the obligatory tuk tuk. For us, 4 nights was perfect which allowed a full day trip out of the city plus not a hurried pace. Some people had said 2 nights is enough but it really wouldn't have been for us.
Had been worried about Koh Samui as it was described as built up and a bit 'like Spain'. But we stayed in a fairly remote hotel in the north - or it felt that way and nearest 'bigger' town was the Fisherman's Village 15 mins away - and it was incredibly green, lush and empty and with a wonderful private beach overlooking Ko Pha Ngan(which was described btw by one of our waitresses as the party island though it is supposed to be less developed).
We did a boat trip one day which took us along the coast (where it seemed most hotels were low rise and these villa type buildings on stilts so most with sea view - and blended in beautifully so not at all like Spain there for sure. We then went to Pig Island, followed by snorkelling (basic there, not really known for it but fun) before they took us to an amazing, quiet restaurant right on the beach on mainland Samui. It was amazing.
We didn't explore really on Samui but did do a cookery class at the hotel. We loved our resort and Koh Samui.
Weather wise, we had been a bit worried but we were perhaps lucky. In Bangkok it was very hot and humid (but we're fine with 30 degree heat) but there were only 2 very short showers during the daytime whilst we were there, but so light that our raincoats stayed unpacked. Koh Samui - didn't rain once during the day but every night/late evening (same as in Bangkok). I had been worried as the weather apps had said rain before we went and it said the same whilst we were there but it just didn't materialise, not even a quick afternoon shower which we would have been OK with. Saying that I think Samui was unusually dry this year.
All-in-all a wonderful holiday and experience. We'll be back!