It has been decades since I was there, but when I visited Pai in the 80s there was a lot of partying going on. Personally I didn't enjoy it for just that, but the scenery was gorgeous. On the other hand, many, many others still love the place. In hopes that still holds true, you could start in Chaing Mai, a good place to met other travelers. Explore the area around it. Then finish up in Pai for a few days to party before heading back to Bangkok and home. Or do the Mae Hong Son Loop finishing in Pai and then Chiang Mai. This last itinerary could be a bit rushed, but certainly possible in 16 days.

Or head south to the Andaman Coast and finish up on an island/beach in Krabi or Trang for party time, Koh Lanta might be a good choice, then back to Bangkok.

Thank you very much for all the information. Very helpful.
So to summarize, it's either the international bus on Mon, Wed, Fri or Sun for THB1.500. But can't book in advance.
Or fully travel by local public transport which seems relatively cheaper (CM to CK THB202, crossing THB25, tuk-tuk KIP25,000 and bus to LP KIP130,000, a total of about THB800). I suppose we do not book that in advance either.
Finally, I recently found this "tour bus" which also goes from CM to LP for THB1,450.
https://www.travelhubchiangmai.com/tour-detail/tl1001-chiang-mai-to-luang-prabang-by-bus.html
It seems that they run every day, and are competitive in term of price. And I can also book in advance. Have anyone heard about them?

Quite frankly if you have been to Thailand 3 times already I am surprised you need to ask this

if you have your tent why do you need hostels/guesthouses

I would suggest Narathiwat, Patani or Yala provinces very few tourists and easily eat for 20 baht a meal (chicken rice, broth and salad as well as numerous

I have read that Doi Khun Than is accessible by train. Ca. 60km South of Chiang Mai.

Soren

Hostels are not so popular in Thailand many cheap guesthouses and hotels

How much depends on what you will do apart from eat drink and sleep as well as the basic costs. bangkok will be more expensive than Narathiwat and so on

I think Koh Rok would be a great destination for Xmas. However, if you plan to be on Koh Rok for the holiday and Chiang Mai for New Years and want Mid-Range accommodation there, you should be booking now. You may already be too late for camping on Koh Rok. We were on Koh Lanta 2 years ago in late March and we were trying to get a few days at Koh Rok. The tents were fully booked for the entire 7 days we were on Lanta and we were checking 2 weeks before we got there. I can't imagine it any different in peak Xmas season. For transfer, I think you will have to get a ferry from Koh Yao Noi to Koh Lanta first and then a boat from there to Koh Rok.

Yes, of course, it can be advantageous to get info. Younger, much younger, that is what I did, like everyone, and it is true that having one's own apartment can be lonely and disconnected. But I find hostels very uncomfortable and I only ever stayed in them in Latin America precisely for - the kitchen use!!! How many tortillas can you eat, anyway? :) A good compromise is to find hosts who live onsite and who can inform and aid you. That is what I have tried to do here, and it was a good plan. But I need my own calm private space. And I might not necessarily be looking for the same thing as most young travelers, things change with the years. :) And often young people in hostels can be hostile (could be a play on words, not intended haha) to older people, they are traveling to get away from their parents etc. At least in Latin America. But I think in Asia too, I have heard that. The partying can be fun for them - I have seen groups just decide, for example, settle into drinking all day - but not for me. Shared kitchen but I was not part of their clique and hardly welcome, although I said nothing. Plus sometimes your food disappears from the crammed shared fridge. So there we are. With airbnb you can certainly find kitchens. Another solution is a room in a home with local people, this can be nice. This works better if you can speak the language. But I do have my own rhythms and needs. Once upon a time I saw that there was movement for senior hostels - this could work well in Franc, for instance, where they love to wander around a lot - but it seemed to have faded. Maybe not enough demand. and too many issues. Thank you for the input, anyway. And have a god day.

I would suggest the Tongo point area for the turtles and some nice corals . It is more of a chilled place to snorkle but I have not seen the sardine schools there.

The sardines can be seen at times along the shores in front of Panagsama in general

The road from Moalboal market to Panagsama makes a left turn just past Eves Kiosk and Nelsons Dive shop. I used to enter the water there and would often see sardine schools swimming right off the beach near there. Best to carry your own mask and snorkle but fins and masks are likely available for rent from the guesthouses.

There are a couple of hostel type accomodation on the road to Panagsama and I think there is a hostel just past the main parking in Panagsama on the left side of the road.

If you were interested in snorkeling with whalesharks you could organize a daytrip from Moalboal to the Tanawan Oslob whaleshark feeding beach. It is a bit artificial but might be kind of interesting . The whalesharks are fed small shrimps by the handful and stick around for the selfie crowd. Maybe take a group tour from Moalboal and be there by 6am and back in the afternoon.

Also White Beach near Panagsama is an actual beach that you might find interesting