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.
Unlike Bangkok, which has warm evenings and blazing hot days pretty much all year, Chiang Mai cools off nicely during December, at least in th evenings. This is an outdoor city with an emphasis on hiking and exploring temples so the cooler temperatures will be welcome for most people. It's still hot most days, and quite dry as well, so it balances nicely.
If you are interested in a place that isn’t focused on beaches in that area then I’ll point you to some recent answers just above where I mention the charms of Chiang Mai in northern Thailand, or Luang Prabang in Laos, and/or Siem Reap in Cambodia. All of those are wonderful places with temples and other sights, and they are quite affordable once you are there as well.
Actually, all of my best suggestions for those things are in the article above. One challenge you’ll have is that all popular tropical destinations have their peak weeks at the end of December because so many people in cold areas have that time off. So flights to Central or South America are going to be fairly expensive, although buying soon can help. For hiking and views I think Costa Rica could be a good choice, or Nicaragua. You could also go to Bariloche in Argentina, though it’s probably not worth going all that way unless you were also going to spend some time in Buenos Aires and such.
The second trip will be the second week of December from the 13th to the 19th. Also warm place with adventure and tourism. Im going with my best friend. I was thinking one of the islands of the Caribbean or Latin America ( since I can travel freely to most of them without the process of getting a visa) but I’m not sure which ones where not affected by Irma.
Assuming you might be in the US or Canada, my first hunch is Costa Rica, or Panama, which is a bit cheaper though not as well organized. Costa Rica has good surfing on its Pacific beaches, and there are very few all-inclusive resorts, which means that the restaurant scenes in resort areas are vibrant and interesting. Costa Rica also has limitless adventure opportunities, like zip-line tours and such, although they aren’t exactly cheap. Going inland you’ve got loads of national parks, as well as the famous volcano and its little spa town.
Traveling during the last ten days of December means crowds and exorbitant prices throughout much of the world, but the first two-thirds of the month—before holiday airfares and hotel rates go into effect—can be a serene, value-laden, and just plain lovely time to vacation. Europe is all decked out for the holidays—with locals generally friendly and in high spirits; in the Southern Hemisphere, spring is in bloom; and in the Caribbean, Hawaii, and Mexico, hurricane season is over, the weather is gorgeous, and there are free upgrades galore.