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.
Sri Lanka could also be great, and it’s very different. In fact, you might even consider a trip to India itself, which is obviously similar to Sri Lanka. But Sri Lanka is far easier to do in about 12 days, with most of your time spent in the mountains and a bit along the southern coast. Colombo is mainly just good for flying in and getting your bearings. Sri Lanka is also a bit more expensive than Vietnam, but both are great value. And again, you could instead go to Kerala and Goa in India on a short trip, although those are more about relaxation than culture. Let me know if you have any other questions, and I hope this helps. -Roger
From December through February there are literally no European cities that could qualify as having “great weather,” except for the Canary Islands, which is listed below. So during the winter Europe is all about cultural tourism, and lower hotel prices make it especially appealing for those who like to stretch their travel budgets and avoid crowds at the same time.
The rainy season in Cartagena winds down by early November, so this is an ideal time to come, especially toward the end of the month if you can get cheap airfare before and/or after Thanksgiving. The temperatures are pretty much the same here every day of the year, so the key is to work around the autumn rainy season to when it's dry but before hotels fill up.
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.