The December weather is pretty much perfect and dry, and airfares and hotel rates are very reasonable up until Christmas itself. If you are in California you might also consider Los Cabos, although Puerto Vallarta is more interesting and usually cheaper as well. This is definitely one of the best places to go in December if you are anywhere near the Pacific Ocean.
As for Costa Rica, I think it would be ideal for what you have in mind. It’s by far the most tourist-friendly (and English speaking friendly) country in that area, so it’s a great starting point. Belize is pretty easy as well, although there is far less nature to enjoy. Panama and Nicaragua are a bit cheaper and have similar things to offer, but Costa Rica has the best infrastructure and nature parks, so it’s less stressful for a first-timer. The snorkeling is better on the Caribbean coast, I believe, but the better tourist beaches are on the Pacific. The country is pretty quick to get across, so you could easily spend a few days on both sides, plus another few exploring the rain forests and volcanoes and such.
Another reason to consider Dubai is that the busy airport has cheap non-stop flights coming in from almost every major airport in Europe and Asia. As a result it's popular to book long weekend trips of 3 or 4 days to Dubai, where you can enjoy some sunshine and shopping before flying back home where it gets colder by the day. You might also consider stopping for a few days in Dubai and then moving onto somewhere else with cheap direct flights such as the Maldives.
People don’t think of Moscow for winter, but it’s such a wonderful time to be there. The weather is no worse than what you’d find in New York City, and the huge advantage is the people—or, really, the lack of people! No masses of tourist buses descending on the Kremlin, no huge lines for museums (of course, we ensure our travelers skip all museum lines anyway), and no crazy crowds in Red Square. Something magical happens in Moscow in the off-season: It actually becomes more approachable and less intimidating. (Plus, December falls in the heart of the theater season, so for culture vultures this is the best time to visit.)

I like your comment about how “rainy seasons” are almost always overstated. I totally agree based on my own experiences and research, although there are a few places where they should be taken more seriously than others. On my most recent December trip to Bali (2.5 years ago, I think), I spent 30 days there and the rain only became a problem that one time I mentioned. It was just an insane cloudburst while taking a minibus from Kuta to Ubud, and in the end it’s actually probably my best Bali story. Aside from that, it’s very humid during Bali’s wet season, but even that really didn’t slow me down. The thing is, in Bali you are usually on or near the beach or a hotel pool (feeling the breeze) or in your air-conditioned room. Or you might be touring around to see temples and rice terraces and whatnot, and everything is outdoors and beautiful. When it does rain it’s usually over in 15 minutes, but often the rain comes over night. That kind of humidity can be a killer when visiting a city, but for me it’s a small issue on a tropical island like Bali.

December is an ideal time to visit Mendoza in any year: The grapevines are alive, and the backdrop of the majestic Andes after the winter snows makes for dramatic scenery. Wine season is in full swing, and there’s a beer festival in Godoy Cruz during the first week in December—a four-day street party with live music and heavy partying. (Christmas and New Year’s, though, are high season for hotels, and the wineries close on December 24, 25, 31, and January 1.)
Last month we had a week long vacation to Sardinia. You can watch the vlog here. I managed to stay keto the entire vacation, without too much effort. It was quite easy to stay keto actually. I did give myself a talking to before we came and I decided that I was going to do a totally keto vacation to show you guy that it absolutely can be done.  I thought that I was going to fail at the first hurdle when we got to the lounge in the airport. It was all carbs.
Since flights to Mexico City from the US and Canada are usually quite cheap this time of year as well, this could be a perfect short cultural holiday. This city has great shopping, dining, and nightlife, and the central historic and tourist districts are far safer than most people would expect. There are ruins just out of town as well, so a bit of everything is available.
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