As for key events in December, I typically avoid those sorts of things myself, so I’m not a good source on that. I actually visited Rio de Janeiro during Carnival, by accident, and it drives hotel prices so high and also crowds that it’s probably the worst time to visit unless you specifically want to spend all night watching the parades. And Oktoberfest, as another example, literally triples the prices of hotel rooms in Munich, so it’s only worth it if you REALLY want to get involved in the Oktoberfest thing yourself. But plenty of other travelers love to go witness the biggest events, and I assume they love it, so there is no right or wrong.
Regarding Thailand, I’m not familiar with any islands that are beautiful and not touristy. Phuket has beaches all over the island and many of them are isolated so the only guests are those staying at the resort in front of them. If you search other travel sites, including travelfish.org, which I think is the best site for southeast Asia and it’s run by a friend of mine, you can find more information about more obscure Thai islands. One challenge could be that you might have to take two or three total flights and then a ferry ride to reach them.
Mid- to late-November welcomes warm, dry sunshine with temperatures of about 80 degrees in this subtropical Central American nation located south of Mexico. You’ll slip in before the holiday crowds and enjoy exceptional weather and affordable prices for tours and fine lodging. White sandy beaches, friendly native people, jungle adventures, and tours of ancient Mayan ruins await visitors on this small and easy to navigate paradise. Nov. 19 is Garifuna Settlement Day, a celebration of the Garifuna people, descendants of African slaves and native Carib islanders. All are welcome to participate in dancing, parades and tasting traditional foods.
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.