Indochina trip in 2020

My only time in any of those places was 2 separate nights in a Bangkok airport hotel 22 years ago, flying to/from Nepal and the USA. I walked down the street to a pharmacy one morning for an attempted transaction using charades, before my Kathmandu flight departed, and that was the extent of my Thai time.

My former dentist, 5 years ago and a year before he retired, went on a Vietnam tour trip with his wife, and sounds like they enjoyed it. Despite having traveled independently myself several times in Europe and on part of a recent Morocco trip, for a 3-week trip involving the cities you mention I’d probably opt for an organized tour. Unless you have connections there already, or some experience with those cultures and their languages, I’d guess that traveling independently, even with some guidance, would be a challenge - but could be really rewarding. Happy travels, however you go!

These are common countries to visit on the backpacker route. A lot of them speak english and get by just fine independently. I believe these days, it is referred to as SEA or Southeast Asia.
Sarah Murdoch, Rick's guide, is branching out and doing tours on her own of SEA. Here is a link to her tours page https://adventureswithsarah.net/tours-with-sarah/
I think whether you want a tour or independent depends on what you want to get out of it, your travel style and travel confidence.

Personally, I would prefer to organise my own trip, as most organised tours rush too much. However, there are many companies that offer tours or suggestions for tailor made holidays, which will give you itinerary suggestions and what to see where.

The time of year that you can travel is key, as the climate varies significantly between these locations.

We hope to go in the USA winter of 2020 January or February.

These places are really easy to go independent and you will save so much money doing so.
I used Jason Super Star service for the Mekong Delta area. He was great and custom made the tour to what I wanted to see. Flights are extremely cheap between the different areas.
I also found the sight www. travelfish. org very helpful and they have a very active forum.
Spend a couple of nights in Phnom Penh if you can too, will break up the journey nicely. We took buses between Ho Chi Minh city/Phnom Penh and Siem Reap. They were very comfortable, cheap, clean and modern.

Jan-Feb is the best time for those places, weather-wise. You can plan the whole thing on your own and find local day trips and guides with cars as needed. You will need to get a visa in advance for Vietnam or a VOA. Distances are long and land transportation is slow. You should plan to fly between cities. There are several reputable airlines offering budget fares.

Saigon - good markets and a few interesting sights, especially museums. The days I enjoyed most were day trips, one to the jungle of the Mekong Delta, the other to the Cần Giờ Biosphere Reserve (mangrove forest) and much more including a birds' nest soup factory.

Bangkok - love it! You could spend two weeks here and not get bored. The temples and palaces are gorgeous, very good museums and markets - especially the flower market. I've never gone to the floating markets, seemed too touristy. 3 visits, I've always stayed here. It's kind of funky, staff are always friendly, helpful and knowledgeable. It's off the tourist beat, in a well-located mainly residential area. No luxury, but great prices, AC, very clean, and even a coin-op washer and dryer. It's a short walk to the river ferry which got me to most of the places I couldn't walk to. I did use taxis sometimes. They aren't expensive and getting into a car with the AC blasting is very nice. If you're going to Siem Reap, don't spend time going to Ayutthaya.

Chiang Mai - wonderful! It's in the northern mountains, so it's not as humid and it's usually refreshingly cool until mid-morning. The temples aren't as grand as in Bangkok, but still beautiful. There's a variety of day trips including nature hikes and visiting an elephant rescue preserve. Please don't go to places that exploit the animals. I think the annual flower show is the first weekend in February. Been there twice, well worth the somewhat higher prices to see it.

Siem Reap - I got an E-visa in advance. At the time, I had to pay a $25 exit tax at the airport on departure. I did a lot of research about dos and don'ts which I'm happy to share, most of it probably still relevant even though it's been 9 years. I arranged a private guide with a car and driver in advance. It was affordable and the best way to see the temples and other sights. The air-conditioned car was a blessing.

I took cooking classes in Chiang Mai and Hanoi. I'm sure you can find them in Bangkok and Saigon.

You'll want at least 5 nights in Saigon (4 full days) for 2 day trips, 3-4 nights in Siem Reap (I had 3 full days, it was enough). Split the rest between Chiang Mai (2 for the town plus as many day trips as you want) and Bangkok no fewer than 4 nights.

PS, it's usually called Southeast Asia.

20 days is not ample time to see all that you have in mind based on many trips I have taken to SE Asia. Of course if this is your one time ever visit to SE Asia then a tour might be ideal for you. I have never used a tour. Viet Nam was easy breezey to do on our own for 30 days. We enjoyed Sapa in the north and Halong Bay of course. Siem Reap is stunning, Stay 3 days to see and absorb this world wonder. Northern Thai is perfect for non beach treks and foodie pursuits. Thai beaches are great for relaxing but I don't think 20 days will do it.
Food is deelish and cheap. Lodging was reasonable. So was transport...ENJOY!

If you like to see floating markets/villages, you have 3 main options: Cat Ba Island and Mekong Delta in Vietnam or Tonle Sak in Cambodia. Personally, I would recommend a boat tour in Lan Ha from Cat Ba Island. The views are just incredible and the floating villages are very impressive, but not so touristic as the Mekong Delta.

For cooking, you have to be in Hoi An. Plenty of good cooking classes to choose from. Just check out TripAdvisor for the best rated ones.

If you like nature, Ha Giang, Ninh Binh, Phong Nha and Kampot (Cambodia) are my favourite. Lots of mountain views, waterfalls and green surroundings.

Please don't try to add North-Thailand, as 20 days is already really short for visting these 3 countries.

Travelling indepently is quite easy nowadays. In Vietnam, there is a good train network and otherwise there are always many bus companies you can travel with. If you want to get from Hanoi to Hoi An (cooking class), this page can tell you how to get there by bus, train or flight: https://www.geckoroutes.com/vietnam/ho-chi-minh-to-hoi-an/

In Cambodia, you have to rely mainly on buses and I recommend to stick to: Cambodia Post VIP Van, Mekong Express and Giant Ibis Transport. These have the best safety record.

See here an example itinerary you could use:
Hanoi 2 days
Ha Giang or Sa Pa 4 days
Cat Ba 3 days
Hoi An 3 days
Phnom Penh 1 day
Kampot 2 days
Siem Reap 3 days
Bangkok 1 day

I think Nepal is good choice, if you want to focus on floating markets, food markets, food tours, cooking classes, natural sights. Here in Nepal you can able to enjoys various natural places like Rara national park, upper Mustang, Annapurna circuit, Manaslu conversation area, etc. Nepal is multi-cultural, multi-caste living people. So, here you can enjoy various types of food tours, cooking classes, food markets, etc. And also it is affordable place. For more you can also visit here.

We are basically independent travelers but took a Mekong river cruise with AMA Waterways with a land tour option. We flew into Hanoi four days ahead of the tour starting date and explored the city, hired a guide one morning. Hanoi, Halong Bay( overnight on a boat, wished it had been two nights) , then to the river boat which held 100 passengers, 50 of them were with an Australian tour group, fun people. We saw places we could not visited on our own. Lots of free time. All meals were a choice of local cuisine or Western. We always chose local.
We stayed in Siem Reap several days and visited all the temples, spent several nights in Phenom Penh, ate dinners in the city on our own, highly recommend Romdeng, walked around the city, markets by ourselves. Ended in Saigon where we extended for several days independently before flying to Bangkok for five more days on our own. We set up a few private tours in Bangkok.
AmaWaterways provided a good experience, much, much better that the two Viking River cruises we’d taken in Europe. We loved this trip.

You could easily spend the entire time in Vietnam. Hanoi is the most interesting city in the country. There are lots of foodie activities in both Hanoi and Ho Chi Minh, hands-on cooking schools and walking tours in search of markets and restaurants.My experience is that you get what you pay for so avoid the bargain-rate programs. The Internet and cell phones are everywhere so DIY arrangements are fairly easy. The only hazard is that travel agencies with good reputations may be imitated by less trustworthy competition. Your hotel will recommend day trips etc, no doubt for a consideration by the operators.

Thank you for feedback.
After researching This looks like what we would like in the tour.

Vietnam (one week)
Ha long bay
Ho Chi Minh
Hanoi
Mekong flouting market
Cooking classes and food tours and markets.

Cambodia (3 days)
Siem Reap
Angkor Wat temples
Cooking classes and food tours and markets.

Thailand (5 days)
Bangkok
Chiang Mai
Cooking classes and food tours and markets.

Any thoughts on timeline adding days etc.
thank you