Was thinking of this question too, thanks.
City area - I think normal T shirt will do.
Mountain - add jacket, maybe add long pant
I'm going late Jan, wondering when is the coldest time.
I think end dec to mid jan could be colder, i asked the locals they say this year not as cold.
in terms of markets and shopping malls these are the ones i think are good
1. Jing Jai weekend market (morning): good selection of local produce and handicrafts, food, chill vibes
2. Ba Pao Flea Market (day): coconut plantation converted to market, smaller scale compared to Jing Jai
3. Saturday walking street at Wua Lai road (night): medium size night market
4. Sunday walking street at Tha Phae (night): large night market with smaller streets branching off. Some of the stalls seems to come from Wua Lai (just roving around the different night markets)
5. Central Festival Chiang Mai: Biggest mall i think, but feels a little similar to SG malls
6. Maya Shopping Center: less shops, but nimman area at night (one nimman, thinkpark) has many stalls to walk
7. Central Chiangmai Airport: a surprise despite old building, the shops suit us better in terms of things to buy
I think staying at Nimman area is good for the night market, unless you going for the weekend night markets then stay at old city.
Grab is good, I travel mainly via Grab.
Grab food is also alright, i ordered mango sticky rice at night when we were hungry.