When you mention, “not too touristy”, the first thing that comes to mind is San Juan, Puerto Rico. I’m not sure if you’ve been there already or not, but one great thing about it is that the “tourist” areas close to Old San Juan are filled more with apartments than with hotels, unlike Cancun, where it’s nothing but hotels and timeshares. I can highly recommend the Condado neighborhood, which does have some large hotels but is mostly apartments for expats and seasonal visitors. There is even a free bus that goes from there to the heart of Old San Juan, which is a gorgeous colonial town filled with great restaurants, bars, and interesting attractions.
As for alternatives you might consider Argentina, where of course it will be late spring in November. Buenos Aires is a wonderful city and recently they’ve been going through a financial meltdown again so their currency has dropped a lot, making things unusually cheap again. You could spend 3 or 4 days in Buenos Aires and the rest in the Mendoza wine country or Iguazu Falls or Bariloche for some adventure. From everything I have heard it’s all safe and civil there, so it’s just a matter of the local currency going down in value.
If you are looking for beach weather in December then most of your list won’t work, and several of them are only on rivers. Starting from Kolkata you have many good choices that are much closer to you than anywhere in Europe or the Canary Islands, or the Caribbean for that matter. In fact, I’m a bit confused by your list because there are so many different types of places on it, many that aren’t warm or on beaches.

Heading east, India could be a possibility, although it’s not for the faint of heart. As a resident of Singapore, I assume you know a fair bit about India. Outside of Mumbai, it’s all quite cheap and the weather will at least be good. Personally, I love the southern state of Goa, and it has everything you are looking for, without the insanely crowded conditions of India’s big cities.
Say Hydrated – This is a no-brainer and one of the most significant challenges for me. I never drink enough water, but it was hot this week in Florida when we were at Disney, and I could tell such a difference in how I felt when I was hydrated. I felt like it made it easier to stay strong and stick to eating low carb/keto as it helps to quench false hunger.
Another option is Chiang Mai, Thailand, which has wonderful weather in December as long as you don’t require constant heat. It is popular with tourists and expats, but it’s not an overly touristy city so it never seems too crowded. Other options in that general area to consider are Luang Prabang, Laos, and Siem Reap, Cambodia. Let me know if any of these sound interesting and I can provide more info if you need it. -Roger

Costa Rica, as you probably know, is all about nature and adventure sports, along with some decent beaches. It would be perfect if you are up for that sort of thing. If not, Santiago and Valparaiso (nearby cities in Chile) could be good for more of an urban adventure. Or Buenos Aires is even nicer and more interesting (and probably cheaper if you bring cash) if you can get a deal on a flight there.
Rio is probably the most beautiful city in the world as far as the setting is concerned, but it’s also famous for petty crime. As you mentioned, I think it’s quite easy to stay safe as long as you are aware of the situation. The biggest problem by far is tourists getting robbed by someone holding a knife or sometimes a gun, and they are often in groups. However, this almost always happens to people who are walking close to the water on the beaches after the crowds have gone for the day. The sidewalks along Copacabana and Ipanema are brightly lit and safe, but there is a mound of sand between the sidewalk and the water, so if you go to the water you can’t be seen by the security guards and police on the street. As long as you avoid that, and any other situation where you are alone in a dark place late at night, you’ll be fine.

If I were you I’d fly to Bangkok and stay at least 2 or 3 nights in the famous Khosan Road area, which has a bad reputation with some people, but it’s really fun. It’s also cheap, and a short walk from most of the main cultural sights. After that you could fly to Ko Samui or Ko Phagnan or Ko Phi Phi, and easily find the better party spots. You could also go to Phuket, but I prefer the others for younger people. Phuket has loads of nice beaches for families and couples, but the one “party town” (Patong Beach) is a bit over the top, and mostly geared for groups of men who want to go to the “beer bars” and strip clubs and whatnot. The smaller islands will have a more mixed crowd, including many backpacking females. Hopefully this helps. Let me know if you have any other questions. -Roger
I am planning a 12 night or close to 2 weeks holiday from London for the last week nov/or first 2 weeks of December to avoid the holiday season. Looking for a sunny holiday so nearest bets are a minimum 7 to 8 hr flight from here. I had few things on my mind mix a wildlife safari and beach holiday in Tanzania (but the safari’s appear v expensive!)or travel to south/central America I haven’t been to any part of south America and was considering costa rican region. Ideally looking for few days on a good beach snorkelling etc and few days exploring the country and nature. Do you think costa rica is a good starting point if it’s the first country I was visiting in south America ?

If you’re a bit more on the timid side, you can always use Yelp and TripAdvisor to get good insights on places people have visited. As a disclaimer, some of the ratings may not be accurate on these sites, so make sure you go through and read some of the more recent reviews that people have written. We did this for a few of the places we visited and had no issues.