I was about to recommend Chiang Mai in Thailand too @systaran. I visited with my mother when she first retired and Chiang Mai and the trips we did from there were a real highlight. We spent Chinese New Year having a meal with a family from one of the hill tribes in that part of Thailand who originate from China, so it was a special day for them. All the family had gathered, including the nephew who had joined the police force in Bangkok.
I also visited Bangkok and Phuket with my mother and internal flights were cheap.
On a separate trip I visited some of the islands. We didn’t go to Koh Samui, but I really liked Koh Lanta, which at the time was very rustic outside the main town. This was over 25 years ago though.
I’d definitely recommend visiting north, south and Bangkok.