What are the best places you would recommend seeing during the month of December over a span of 2-3 weeks?
A lot depends on the interests of the travelers.
My No. 1 priority is weather decent enough for me (cold-natured) to enjoy being outside, given the dearth of sunlight at that time of year. So I'd suggest looking first at southern Spain and second at southern Italy.
Lots of people love Europe in the winter. Tours of the Christmas markets (many of them in Germany) seem very popular. In general, most folks recommend cities for winter trips, because they remain lively even if the weather isn't good. Sights in small towns may be closed completely or open only a few days a week.
We have twice traveled to Italy in Dec/Jan and had wonderful trips. The first time was to Rome, Florence, and Venice. Venice was very cold, but the other two, not too bad. Sicily was wonderful, with mostly daytime highs in the 50’s.
The upsides of travel at that time were the festivities that accompany the holiday season and the absence of crowds. Many churches have wonderful creche displays, and town centers are all decorated. Also, hotel prices are lower than during high season.
The only slight problem is that if your trip extends through Dec. 24 - 26, you need to plan for meals ans attraction closures.
I’ve made 3 trips to Italy in December and loved the absence of crowds. I’ve spent Christmas in Florence, Rome and Sicily. One trip started in Sicily and I drove up to Sorrento and spent some time in Positano before joining a Rick Steve’s Christmas in Rome trip. I visited relatives in Bavaria before spending a week in Como and then Best of Florence over Christmas. My last December visit was taking Best of Sicily in late November and then visiting Naples, Amalfi, Paestum, Orvieto Tivoli and Rome. On all those trips, weather was beautiful. No rain, comfortable temperatures. Oh, did I mention hardly any tourists?
Also spent one Best of Paris Christmas that was capped off by an unusual late December snow fall. A bit chillier than Italy but the city was decked out into a wintery Christmas wonderland.
Why not go north, enjoy the winter and watch the northern lights?
I lived in Augsburg, Germany for four years and couldn't wait for the long cold season to end in Spring. We usually went to Italy then and loved the warm.
Sorry, I don't pay to fly to Europe to freeze or even be just cold.