I note your comment "We both like non touristy areas". Phuket would be the most "touristy area" in Thailand. And the most expensive. I still like the place but be aware that in December the tourists probably outnumber the Thais in Patong.
However, I also note that you are driving from Bangkok, and that on your previous trip to Thailand you loved a place that was "the kind of place the locals went on vacation".
So there are a number of places on the way to Phuket that you would probably like. Firstly Cha Am (only a couple of hours from Bangkok) is quite fascinating (and busy) on the weekends, because so many Thais drive down from Bangkok for a weekend break. It's worth staying the night on the beachfront there to see the kids enjoying themselves on the beach while their parents get on with the more serious task of eating (and drinking) under the shade of massed beach umbrellas.
Then further South the town of Prachuap Khiri Khan is one of my favourite places in Thailand. Stay a night on the seafront in the Hadthong hotel with terrific ocean views from your balcony, have dinner at the street stalls next to the park (try the delicious local specialty of deep fried sun-dried cotton fish, [ask for "pla samlii daet diaw" ]) and next day visit Ao Manao where you can have a relaxing lunch served cheaply on the beach under your umbrella.
Then continue South and given your interest in Scuba diving, you should stay in Khao Lak, because from there you can catch ferries to some of the best scuba sites (try researching Similans and Surin Islands here on TA).
Khao Lak would also be less touristy than Phuket, and yet is less than an hour and a half by car from Phuket International Airport . You can also satisfy your liking for "jungle trekking" close to Khao Lak in the Khao Sok National Park.
Considering your likes/interests, I would go to Phuket, but I would spend a significant part of your 2 weeks staying at places on the way, and I would stay mostly in Khao Lak rather than in Phuket.
Chiang Mai is a nice place with lots to see and do, but I am not as familiar with the route by road between Bangkok and Chiang Mai, so I can't really help you with things to do on the drive.
Hope that helps and gives you some more things to research.
Mike.