What are the best travel destinations in Thailand for nature lovers during late March to April 2024, considering the burning season?

I'm planning my second trip to Thailand in 2024, arriving on March 25 or 26 and flying home on either April 18 or 19. I was planning to go there about 3 weeks earlier, but my Thai friend who lives fairly close to me here in Australia just talked me into being there and experiencing Songkran. I'll be spending my time again with my Thai ladyfriend who lives in Nakhon Sawan Province, the same village that my Thai friend in Australia is from. The last 7 or 8 days, I'm sure we will be in Nakhon Sawan, with her family or close by.

That leaves about 16 days where I would like to take my ladyfriend to another part of the country and this is where I'm looking for some suggestions on where to go, places to see and some recommendations for good accommodation at mid-level prices with locations you would recommend.

A couple of places that I would like to go next year, or sometime in the future, are Chiang Mai, Chiang Rai and Kananchaburi. I have thoughts of maybe two or three days in Bangkok to see it's highlights, maybe a Market or two, but I'm not sure about that because my ladyfriend, I think, has a vote that the city is too chaotic, but without asking her again, I think that was more to do with driving in the capital. Last year I visited The Grand Palace on my last day in the country.

While I would probably like to see more of Bangkok's highlights, I don't want to spend much time in the hustle and bustle of big cities or crowded tourist destinations and being from a village, I think my ladyfriend would feel the same. Neither of us are the beach or laze around the pool, or Bar and Nightlife types. I think 'nature' would appeal to both of us a lot more than cities.

Chiang Mai, Chiang Rai and Kananchaburi, for different reasons, I'm not sure about going to at that time of the year. The former 2, because of smoke pollution from burning off crops. Are they best avoided in the time period I'll be in Thailand? Kananchaburi, checking out the weather in these 3 places, seems like it is the hottest and most oppressive time of year (I will be coming out of the Australian summer where our average temperatures are in the high 20's and depending on the year, reach maximums in the high 30's or low 40's on several occasions, but fairly low humidity).

Your thoughts on destinations to go, places of interest to see, accommodation in any places you recommend, please?

Please limit your comments to helpful ones as per requested. Any comments offering relationship advice, or ridiculing/belittling with result in an immediate 'block' from me. I do not treat others that way and won't tolerate that treatment from others.

Thank you