France WWI battlefield tour in December

Cold, wet and probably muddy as well. The problem is that unless you've really done your homework, one battlefield looks pretty much like another. Since you plan to invest quite a substantial chunk of your French holiday in the battlefields, your best bet would be to catch a train from Paris, possibly to Lille, and take a guided tour. One guide who gets consistently good reviews, including many from Australian visitors, is Rod Bedford of The Battlefields Experience . Bedford specialises in day tours but he can also put together a longer customised tour. 

Since you'll be spending a few nights in the area, one hotel that gets consistently great reviews is the Ambrosia ,  a small and charming family-operated hotel in Ypres within walking distance of the Menin Gate, site of the nightly Last Post ceremony which is something you won't want to miss.

Another option is the Spirit of Remembrance , a specialist tour for Australians, with the emphasis on the Australian sectors of the Western Front battlefields of World War I, including Pozieres, Fromelles and Villers-Bretonneux.

A week of battlefield touring might be a bit much, especially in chilly winter. I reckon three days sounds about the most you'd want to spend. 

Colmar is a treat at Christmas time and thanks to its Germanic heritage it makes a big deal of Christmas. For New Year's Eve, it's hard to beat the razzamatazz celebrations that happen in big cities, so Paris.