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I can vouch for Croatia, but i would strongly suggest to go during summer for an unforgettable vacay! You can fly into Dubrovnik from nearly all major airports in EU and then drive up the main highway which runs from most southern coast tip all the way to farthest northern tip (something like PCH in California).. rent a car and drive into every city small and big, swim in crystal clear water, enjoy great food and atmosphere.

You didnt mention what kind of activities you will be doing in Dec.. if you Ski/board i suggest going to the French Alps during those time frames.. fly into Zurich and catch a transfer up to the three valleys.. i suggest staying at Val Thorens.