We also are honeymooning in December!
 

I am helping my son plan their honeymoon in December this year, 2022. They have 3 weeks total, but take off 4 for travel! They would like to visit Phuket, Ko Samui, Wat Phra Kaew, the Krabi Province( do they need to do Phuket and this?) Chiang Mai and Chiang Rai, and Bangkok. I was thinking they could fly into Phuket and then fly to Bangkok, and then fly to Chiang Mai. Maybe they could use buses to explore from there. They like to get off the beaten path, immerse…and then finish with some real luxury. Any suggestions?

At that time of year I would skip Koh Samui as the weather wont be at its best. Plus with limited time they would lose a day in transfers. beaches are arguably better on Phuket and the Andaman coast anyway.

The best way to see the north of Thailand and “get off the beaten path” as far as that is possible these days, would be toffy to Chiang Mai, spend a couple of nights to explore the walled old town and maybe Doi Suthep there and then rent a car. Chiang Rai is a 2 hour drive away. Highlights there include the Black House and White Temple. There is a fantastic walking market on a Saturday night - amazing street food!

Also in the area is Chiang Dao. this is a decent place to stay. not luxury but lots of activities and a little bit OTBP.

Another option we have done a couple of times, is the Mai Hong Sort Loop. Drive out from Chiang Mai - Pay - Soppong - Mai Hong Sorn - Mai Sariang - Doi Inthanon and back to Chiang Mai. A great drive through stunning scenery with a few hill tribe village along the way https://www.travelfish.org/trip_plan...-hong-son-loop

Luxury hotels can be found all over Thailand from beach resorts in Phuket to incredible city hotels in Bangkok so it would make sense to top and tail the trip in one of those. Personally I would choose Bangkok . The Mandarin Oriental, The Peninsula, and the Four Seasons are all bon the river, are all brilliant but expensive. Also on the river are cheaper, but also excellent options like The Shangri La, Anantara Riverside. Away from the river there are many more options

When you say 'we' are you going along? What's the budget per night? If the COVID thing is receding rapidly next December and many tourists come to Thailand i would expect the hotel prices would be sky high, to recoup some of the losses of the past 2 years.
I have not found any less-beaten path for tourists in Thailand. You won't be able to communicate without a guide and he will direct your path.

Great! Good advice! His fiancé is looking forward to Elephant sightings. Is the preserve in Chiang Rai the best option?

I would book refundable hotels now as many still have Covid pricing.

I certainly will not be joining them! They have 3 weeks, and would like to keep close to a budget of $15,000. , but that includes airfare.
I will definitely book some local tours.

Phuket, Ko Samui, Wat Phra Kaew, the Krabi Province( do they need to do Phuket and this?) Chiang Mai and Chiang Rai, and Bangkok.
Given that you have only 3 weeks, this trip does a lot of overlapping.
How many beaches do you need to do?
I'd suggest find one really nice one with a few extra-curricular excursions and that should suffice.
As said above December on Samui can be VERY wet right up till Christmas week.
I prefer Koh Lanta (Krabi) to Phuket but only because Phuket can be very urban packaged and expensive.
So taking only one out of the 3 sun/sea/sand options would then give you time to 2 other locations.
E.g. a few days in Bkk and then somewhere more "Thai" - either up north or west to Kanchanaburi. THere's a lot more to Thailand than islands and Chiang Mai.

how you ravel will dictate how much time you have in each place
BK to Phuket by air is easy enough.
Then a flight up North.
Other places, you might look into car hire.

There are some stupendous resorts around the National Parks, Doi Suthep is nice but well trodden, just about any National park has accommodation nearby or even in it if you can sort out a place
Kaeng Krachan, Khao Sok, Mae Wong - they can be very rural and quiet though.
If you follow th Thai tourist crowds rather than the foreigners, you get to experience real Thai food and a lot more.
Probably the most beautiful place in Thailand is the Thee For Su waterfall near Umphang.
It's pretty inaccessible though.

Kui Buri and Kha Yai are famous for elephant sitings. I'd avoid paces that involve close encounters or worse still, riding on elephants Animal exploitation is rife in Thailand. Most places use language to attract tourist that can be vey misleading - "sanctuary" "conversation" "eco" are bandied about with scant regard for their true meaning.