I think Sri Lanka would be wonderful for 2 weeks in December. The rainy season typically ends early in December, but even when it’s still going it tends to be quick downpours, as you are no doubt familiar with if you’ve been all over southeast Asia. Still at this point there are very few hostels with dorm beds and not even many cheap budget hotels, but at the age of 30 you are probably happier with normal hotels anyway. Value is quite good in Sri Lanka as long as you can spend at least US$30 or so per night, and hopefully a bit more. Food is also quite cheap in general, although there are still surprisingly few tourist-oriented restaurants in many towns so it feels a bit undiscovered. Just don’t spend much time in Colombo and preferably focus on the mountain areas, and I think you’ll love it.
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.