Into Munich and out of Amsterdam at Christmas with Family

"...a car factory or two... a Castle or two... a couple of Christmas markets... worthwhile museums..."

I would suggest stopping in or near Speyer which lies south of Mannheim. The Technik-Museum Speyer - http://speyer.technik-museum.de/en/ - has an awesome display of vehicles, aircraft, and vessels. Also, Speyer has one of Germany's most impressive Romanesque cathedrals - it's a UNESCO World Heritage Site - as well as a Christmas market that runs AFTER Christmas into the first week of January.

Speyer info
Speyer Cathedral
Speyer Christmas market

Be sure to tour Marksburg Castle in Braubach in the Middle Rhine Valley on the way north - it is "unchanged from Medieval times." There are 40 castles in 40 miles of river here as well, some in ruins, some hotels (maybe book one?) but Marksburg is your best tour stop.

Marksburg photo
Marksburg info
Braubach's old town center
Middle Rhine Valley towns

Amsterdam and Munich have good museum choices.

"Weather will be a factor, too, I'm sure."
Currently there's an excellent discount price on the 5-day German Rail pass - €260/2 adults (twinpass.) Using the train is quicker and easier overall - no winter driving chores, no winter tire fees, no parking fees, no $8/gallon gas, no contracts or shady agencies (LOTS of complaints about this in Germany.) The German Rail pass covers the above destinations and about 5,500 other destinations in Germany. It can now get you to Brussels and to Antwerp BE as well if you decide to go there. Antwerp is big on brew pubs. (And of course if you want to sample some European brews, the rail pass is an even better idea.)
German Rail Pass info
GRP Promotional offer

Have you looked into the BMW museum and tour in Munich? It gets great reviews.

If you're doing this by train, which I would recommend, have a look at http://www.bahn.de/p_en/view/index.shtml for what the price of tickets in and from Germany would cost for the four of you, when you book them now for December. I doubt very much whether a rail pass would be of any financial advantage.
There are dozens of interesting places between München and Amsterdam. Antwerpen would be nice to get the taste of wonderful Belgium. From Antwerpen it's easy to have a daytrip to Brussel and/or Brugge. Consult http://www.belgianrail.be/jp/sncb-nmbs-routeplanner/query.exe/en For trips within Belgium tickets are bought at the railway station.
For a trip from Belgium to Amsterdam, you can best use the website of the international branch of the Belgian railways for timetables and reservation: https://www.b-europe.com/Travel/Destinations/The%20Netherlands These are also best bought well in advance.

Have you looked into the BMW museum and tour in Munich? It gets great reviews.

Indeed. The BMW complex (factory tour, Museum and BMW Welt) is excellent and well worth a visit. The Olympiapark stop on the U3 or U8 underground line takes you right there.

They might also enjoy the transportation branches of the Deutsches Museum -- Verkehrszentrum, covering all manner of ground transport (Schwanthalerhöhe stop on U4 or U5); and Flugwerft Schleißheim, for things with wings (S1 to Oberschleißheim, then a ten-minute walk). The Schleißheim airfield itself, by the way, is Germany's oldest operating airfield, and was a military and civilian training base dating back to the First World War. Some of the museum buildings go back to that era.

The main Deutsches Museum campus on Museuminsel is undergoing a major renovation for the next several years, and many exhibits will be closed from time to time. Details here.

Munich's new NS Doku-Zentrum just opened last May, just east of Königsplatz on Briennerstraße, on the site of the Nazi HQ building destroyed late in the war. Adjacent is the large three-story building that was then Hitler's office building (Führerbau) where the 1938 Munich Accord was signed, and now is a high school for theater and music. The DZ's website offers free downloads of maps and audioguides for thematic history walks in central Munich.

We have not done the factory tour of either Mercedes or Porsche, but we did do the museums. The Mercedes Museum was my favorite. It was fantastic. We spent 4 hours there and could have done more. The old cars were amazing, and the museum does a great job of weaving what was going on in world history with the history of the car. Even if you are not a car buff, it is really interesting! I would love any one of those cars!

Porsche was also interesing, we spent about 2 hours there.

BMW Welt is a good choice as well. The Olympic Park is near by too.

"...for the history buffs...." I take that to mean some serious students of history. How about some very esoteric military museums, not the usual superficial coverage. Here the subject matter is well delved into. When I saw one of them, there was then no audioguides, which I don't use anyway, so I don't know whether they would be available now. These two military museums are in Rastatt (a bit far from Munich but still can't be done as a day trip) and closer to Munich the town of Ingolstadt. Both are absolutely "worthwhile museums" assuming you have are seriously interested in studying the history of modern war. The coverage has recently been expanded, one of the largest in Germany next to that in Dresden-Neustadt.

These two military museums are in Rastatt [...] and closer to Munich the town of Ingolstadt.

In Ingolstadt you can also tour the Audi factory. I like Ingolstadt; the old center is quiet and relatively untouristed. Also there is the German Museum of Medical History, in the colorful, baroque structure that once was the old anatomy building of the University of Ingolstadt -- the setting for Mary Shelley's 1818 novel Frankenstein. Gardens behind the building are made up of medicinal herbs and plants.

Thank you everyone for your kind suggestions. Never thought to stay in a castle on the way to Amsterdam....something to definitely look into. Marksburg is a good possiblity too. Looking into the Railpass as you suggested Russ...could be the way to go with the twin discount. Once we are in Amsterdam or Brussels, can we still use that rail pass if we are not traveling back into Germany from there? Also, can you still reserve a seat?

Tonfromleiden, do you think Antwerp is better than Brugges or Brussels? I'm thinking we may have to pick and choose if we want to do Amsterdam justice. Or maybe Belgium has more to offer than Amsterdam?

I also sent an email to BMW Welt and that's looking very promising. They do not book way out like Mercedes does so they will be emailing me when the booking for that date opens up. Good to know the Mercedes Museum is worth going to without the tour. We might take a brief stop on our way to Amsterdam.

Ingolstadt is a place that hadn't even crossed my mind either but my 16 year old knew of it and seemed very interested.

This is a really good start...hoping to have a solid schedule in mind by the end of the week so I can square away hotels. Any other suggests would still be much appreciated. Thanks again.

For a winter day trip, Bruges can be magical on a sunny day. The old pedestrian center is more compact and a little easier to negotiate than Antwerp (which I also like) and in winter there are few tourists to slow you down. There are some great museums, a small brewery to tour, canal boats, bikes to rent, and great beer selections everywhere. Brussels is a big city with widespread sights and takes more planning and focus to find the best of it.

Hi,

If your 16 year old is familiar with the name of Ingolstadt, bravo! The tourists going there are mainly German, certainly not on the American tourist radar. The city is the location of the expanded (within the last couple years) museum on WW I, something to see thoroughly and carefully if you want to delve into the subject matter and how it affected German civilian society in the war years. Nice town to stay in or go stay as a day trip r/t taking in the museum, of course, by taking the early train from Munich.