Every location is best during December. But because its festive month in Western countries, most of the places are crowded and ultimately become costlier...
Hey, we went to Kasmir in December, and believe me, above all those rumours, its a perfect place for travelling considering that you plan it properly. I have posted a detailed itinerary in my trip if you wanna check it out. Have a great trip!
As you know that the month of December is one of the best month for enjoy and if you want to travel so you can go north eastern state of India and specially you can go Arunachal Pradesh ..
Italy and Spain are quite comfortable during December as the weather is not in minus. Also, the tickets are reasonably priced due to a high connectivity between Rome & Delhi and Madrid & Delhi. The Christmas markets make the atmosphere almost magical. You can also consider Bali as the weather is pretty much the same all year round and it works out to be relatively cheaper. Sri Lanka also offers a great package.