Need Advise for Germany in December

Trains are so well organized that if you are just hitting the high spots they will work well. Weather will be wet and cold most likely and while driving is easy, so are the trains without the hassles of getting around and parking in cities.

https://www.bahn.com/i/view/index.shtml

Nuremberg is probably the best known Christmas Market, but there are also nice ones nearby in Furth, Bamberg, Augsburg and Munich.

For museums and WW II history Nuremberg and Munich have extensive possibilities. Munich also has Dachau nearby.

https://museums.nuernberg.de/nuremberg-municipal-museums/

http://www.munich-touristinfo.de/museums.htm

Of course, Berlin is in the opposite direction and also has a Christmas market and lots of WW II and Cold war sites, and excellent museums.

Check out the RS or other publishers Germany guidebooks.

As for flying into and out of Cologne - It is in northwesternmost Germany - if you are not focused on the Rhineland region (which is beautiful and has lots of possibilities too) I would suggest flying into somewhere more central (Nuremberg or Munich if they are your focus, or Berlin if you choose to go there). It will probably be US to Frankfurt, US to Munich or US to Berlin.

Have a great time!

Yes, is there a reason to fly in and out of Cologne? No flights from North America, or are you coming from elsewhere in Europe?

With one week, pick two cities and fly to one and out from the other. More time touring and less time deadheading to where you really want to be.

I would expect a car to be a liability and not just because of the likelihood of snow. Wherever there are Christmas markets, the streets are closed to most, if not all, vehicles. That means you are likely to have trouble with parking. Trains are fast, comfortable and take you from city center to city center. Check for possible passes that would save money.

The only market that I thought was really special was the medieval market in Esslingen, near Stuttgart. I would choose places where you want to sightsee and then enjoy the markets there. Munich has several big markets that are different from each other.

Given that I've spent 33 nights in Berlin over the last 4 years vs 3 in Munich, you will not be surprised that I would recommend Berlin over Munich. I find it much more interesting and vibrant. As far as Christmas markets, I think it is hard to beat those clustered around Stuttgart. There is the large-city Stuttgart market where all the hut owners decorate their roofs festively, the quaint Baroque market at Ludwigsburg, and the super-cool medieval market at Esslingen.

If I had the interests in your list, I would fly into Berlin, move on to Stuttgart and then finish in Cologne (where maybe you are visiting friends/family?). For transport from Berlin to Stuttgart, you can take a 5.5-hour train journey (priced around 30 € for discount tickets right now) or you can take a 75-minute non-stop flight (Eurowings currently selling tickets on a random day in December for 25 €). I would skip the rental car; everything can easily be done by public transportation.

Happy travels!

"My friend would like to go to some Christmas Markets, large as well as some smaller, more quaint ones. My brother would like to see some museums or works of art. I would like to see some WWII history."

So.. if you want a "week where we're not on the road the entire time" - which I think is hyperbole for "not too much road travel" - then you should probably not be in Cologne AND Berlin AND Munich.

Berlin makes the most sense to me with your interests, and 4 days there is just about right.

And you will use CGN airport. That's convenient to Cologne of course. Visit the old Gestapo headquarters, now a Nazi Documentation center, and the Cathedral.

https://museenkoeln.de/ns-dokumentationszentrum/default.aspx?s=315

But Aachen, a much smaller place than Berlin, and closer to what you're looking for, isn't far from Cologne. I suggest you sightsee briefly in Cologne, but stay in Aachen; check it out.

https://www.aachen-tourismus.de/en/discover/sights/

Aachen has a very atmospheric Christmas market:
https://www.aachen-tourismus.de/en/discover/events/aachen-christmas-market/

And nearby Monschau, a much smaller and impossibly beautiful old world town near Aachen, also has a Christmas market as well. Rent a car for a day and drive there if you can.

https://tourismus.eifel.info/2017/11/10/monschauer-weihnachtsmarkt-im-neuen-lichterglanz/

While you have the car, stop by the WW II "dragon's teeth" fortifications outside Aachen:

https://en.wikipedia.org/wiki/Dragon%27s_teeth_(fortification)

Thank you everyone. Some really great info :)