This is a really interesting question. If you want to avoid crowds then you have already figured out that it’s best to avoid Honolulu and I would avoid Oahu completely. The Big Island is very interesting because of the volcano and some of the other scenery, but the most beautiful islands are Maui and Kauai. I haven’t made it to Kauai yet, but everyone I know has been there and they swear it’s the most photogenic, which looks right based on the photos I’ve seen. Maui is also really gorgeous and all islands other than Oahu are uncrowded even in high season. I drove all over Maui in a rental car and loved it and I don’t think there is as much to see on Kauai. So either of those will be great choices and the weather should be almost perfect in December with warm (but not scorching) temperatures and almost no rain. Have a great trip and let me know if you have any other questions. -Roger
While Cancun has become the king of the Mexican beach resort towns in the few decades since it was formed, it's only one of many, and all of them on both coasts have very nice weather in December. In fact, Cancun is probably the most expensive of them, so especially if you are coming from the west coast you are better off in Puerto Vallarta, Acapulco, or Los Cabos instead.
My top recommendation would be San Juan, Puerto Rico. It offers a wonderful mix of history and culture and nightlife and great food, alongside excellent beaches and abundant activities. Hotels and rental apartments are mostly in the mid-range for the Caribbean, but food and drinks and most other things are great value. You could get a place in the Condado area, which is close to the lovely Old San Juan neighborhood and plenty of nightlife all around. And then perhaps you could rent a car and stay for a few days at one of the smaller beach towns elsewhere on the island. You wouldn’t even need a passport, although it does feel fairly exotic while you are there. But because it’s part of the US, it’s also well organized and safe. Most people you’ll meet will speak at least some English, and many are fluent, so it’s easy even if you don’t speak Spanish.
If you book a resort hotel from home you'll probably think Goa hotel prices are much higher than you were expecting, but if you book a family-run place once you arrive you'll still find fantastic bargains. It's probably wise to book a few nights online before you get to India, because it can be incredibly confusing at first. After that you might be able to find a similar place for half the price by just checking around the same area.
The rainy season is over so it's an ideal time to base yourself in Chiang Mai for some outdoor activities in the area. There are temples like crazy in the city center between the old walls, but even more interesting ones in the area just around Chiang Mai. There's also great trekking and such, available quite affordably from any travel agency in the city.
The only places I’ve visited in Brazil at this point are Rio de Janeiro and the Iguazu Falls area. Both of those areas are very nice and justifiably popular. Aside from those, I hear a lot of mixed things about Brazil. I’m sure I’ll explore more of the country at some point, though I’m in no rush. Among other things, English isn’t widely spoken in Brazil (and in Latin America in general). You might know as much about the country as I do, aside from those places I’ve visited.
For our early dinner (Meal 4) we found a burger joint, where I had a bacon and cheese (shredded, not the gross, carb-laden “cheese” sauce) hot dog. I grabbed some green beans with salt and pepper as my side, and sidled up to their topping bar where I smothered my dog with mayo, mustard, onions, and pickles. Dinner was great, and I wasn’t hungry again until much later in the evening (around 9 pm) when the kids and hubby stopped for ice cream. I walked down two shops to the coffee joint and got a plain iced coffee blended with ice, heavy cream, and some sugar free caramel flavor. I accompanied this treat with one packet of my almond butter and was good to go. I sat with my family while they were enjoying their sweet treat and didn’t feel deprived in the least.
Hawaii, as you probably know, would be great for what you have in mind, except that there are no cheaper options there at all. After that I agree with you that there are places in Mexico that could work. One in particular that I like a lot is the area around Playa del Carmen, which is just south of Cancun. It’s a lively and tourist-friendly town, instead of a strip of big hotels like Cancun, and there is a wide range of activities and accommodations in different price ranges.
Prices go up a bit when you go south into Malaysia, but it’s worth it if you’ve got the money. You might even consider Myanmar as well, which I’ve yet to visit, but I keep hearing great things about. It’s honestly the perfect time of year there. By April it’s insanely hot (the hottest month of the year), but December through February is lovely. It’s all quite safe as well, as long as you keep your wits about you. I’m happy to help with more information if you need it.