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.
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.