Need Ideas for 2 nights

Heidelberg is very accessible and enjoyable.
Not sure that the Christmas market starts as early as 22 November, though.
By the way, 4 nights in Stuttgart is a lot! The market there and in nearby Esslingen are very nice, but maybe not for 3 full days. Unless you plan to visit Tübingen or Ludwigsburg as well.

May I ask what you have seen of Germany before? Have you done the "classics" like Heidelberg?

I am asking because what you are planning certainly is not your standard American tourist route. Not meaning to say that your plans are wrong, au contraire, they are just unusual. Or are you maybe visiting friends in those places?

Let me put it this way: If someone were to give me a four-day hotel stay in Stuttgart as a gift, I would certainly go, and I am sure I wouldn't be bored a minute. If I, however, had a gift certificate for four days to spend in any German city, Stuttgart would not be among the first ten.

Freiburg I am sure is a town worth seeing, but I am not sure about three days either. Unless you want to do some hiking in the Black Forest.

Christmas markets usually start during the week before the first Sunday in Advent, and that is in fact the week of Nov. 22- Nov. 27. Provided they take place at all this year. If that is the main focus of your trip, please do check if they are taking place so you will be able to refocus in case some are canceled.

Köln maybe? While the Rhein is mostly traveled in the summer, the towns are still there and just as cute in the winter, and you could visit Burg Eltz.

But I also agree that Heidelberg is nice.

Lots of markets are starting early this year, to make up a bit from the losses of last year and because the number of people that will be let in may very well be limited, using fences, checking vaccines, etc.
Frankfurt had planned on starting the 15th, but moved back to 22 Nov. Heidelberg is starting the 18th. Stuttgart starts the 24th, Michelstadt the 26th, Mainz the 28th, etc. etc.
Go to the websites of the cities to check the dates.

This might be too much train travel, but Erfurt. The Erfurter Weihnachtsmarkt looks like it is scheduled to open 23 November. Had a big opening night when we were there, but with COVID who knows? Gets you out of Hessen and Baden Wurttemberg. A nice city in Thüringen. Will give you a different perspective. We stayed at Pension Harmonie a few years ago. The trams and buses are not too hard to navigate. A lot of really good Stollen from the bakeries there.

A bit south of Stuttgart....how about Tübingen? In 1989 I decided to skip the hostel and stayed 2 nights in a small hotel in the Altstadt....worth a visit, a famous university town with direct train connections to Stuttgart. and scenic.

It's hard to imagine that there isn't plenty of Christmas activity in Frankfurt (and perhaps in some of the many Fachwerkstraße towns.) While Erfurt and Cologne (the latter can't be exhausted in just two days) are great suggestions, they are in the other direction. Aschaffenberg is much smaller, but has some attraction for me.

I am no expert on Christmas markets, but now that you're not going to Luxembourg, why not replace Trier and the Black Forest with something less far afield? Have you timed your train segments yet?

Especially since you bought a round-trip instead of an open-jaw flight, why don't you see if you can get a second segment added onto your Frankfurt (?) arrival. To Leipzig, Cologne, or Nuremberg or even Munich?

The only part of Germany that we've visited is Munich. I know our itinerary is a little off the beaten path, however here are our thoughts:
1) we are car people and are planning on visiting both the Porsche and Mercedes museums (planning on doing them in the morning/markets in the evening) - along with a day trip to Esslingen. We also want to see the art museum and the library, so I'm hoping 4 nights is the right amount (one of the days will be getting to Stuttgart, so it's half a day lost to that).
2) We originally had 2 nights in Freiburg, but changed it to 3 - we're hoping to either day trip to Basel or Strasbourg. We drove through Freiburg a few years ago and loved it and are excited to return to have more than an hour to explore - plus lange rote!!
3) We looked at Koln and Erfurt - but neither made sense with the rest of our itinerary. I'll check the train again to make sure Erfurt doesn't work. The hotels in Nuremberg are crazy high and just out of our budget.
4) We considered renting a car because we'd love to see Nordlingen, Dinkelsbuhl, and Rothenburg but the cost of the car just didn't make sense.

I'll look into Tubingen and Heidelburg and am open to other suggestions. I know Trier is a bit out of the way, but other than Koln, there isn't much that made sense with Freiburg on the way back to Frankfurt.

It's not necessary to have a car to visit Rothenburg and surrounding areas.

Trier is certainly a worthwhile place to visit, being the oldest city in Germany (over 2000 years).

Erfurt would be nice I am sure, but that is really the opposite direction from your route. Heidelberg makes a lot more sense for you.

Straßburg and Basel should be within fairly easy reach, but you'll have to keep an eye on Covid regulations for reentry into Germany.

Have you looked into Speyer? That might also work for you, and it's certainly worthwhile.

Würzburg is an easy trip from the Frankfurt Airport. There is a direct Regional Express train from Würzburg to Stuttgart
using the Baden Württemberg Ticket. It is one of my favorite towns with a castle on the hill opposite town (Marienberg Festung) and a luxurious Archbishop Residenz in the city. You will be too early for the Christmas Market but the town is still great.

https://mcchelsea.smugmug.com/Advent-2010/Wuerzburg/

There is a direct Regional Express train from Würzburg to Stuttgart
using the Baden Württemberg Ticket.

Now that's funny. Würzburg isn't even part of Baden-Württemberg. :-)

Würzburg may be in Bavaria, but the B-W Ticket doesn't care - it will get you there anyway. Like other Länder-Tickets, this one "draws outside the lines," and in more than just one place. Coverage map below.

https://www.bwegt.de/fileadmin/assets/www.bwegt.de/0000_v2/c_PDFs/Karten/bwegt-Streckenkarte-2021-nicht-barrierefrei.pdf

There is a direct Regional Express train from Würzburg to Stuttgart
using the Baden Württemberg Ticket.

Now that's funny. Würzburg isn't even part of Baden-Württemberg. :-)

Würzburg sits on the edge of B.W. so the BW ticket is allowed. This is true in several specific instances. You can use the Bayern Ticket from Ulm to Augsburg and from Salzburg, Austria to Munich.

You can check for these cases on the DB website:

https://reiseauskunft.bahn.de/bin/query.exe/en

Trier is a LONG way to go for very little pay-off, IMHO. I liked the Christmas market there, but it was quite small.

I'm not informed on Würzburg's plans for Christmas, but I like Gary's suggestion of visiting Würzburg. You could easily take a day-trip outing to Rothenburg from there by train.

Also, you make no mention of Mainz at all. It's right near FRA airport. The market will be spread out to 5 different squares in town this year to enhance social distancing. Mainz has several interesting sights and a huge tangle of pedestrianized streets - a great place to wander. See pinks streets on map below.

https://www.orangesmile.com/common/img_city_maps/mainz-map-2.jpg

Since you mentioned Freiburg-Basel, the westerly part of this discussion may interest you, especially if the weather is good:
https://community.ricksteves.com/travel-forum/switzerland/interesting-town-near-swiss-german-border

Trier is a LONG way to go for very little pay-off, IMHO. I liked the
Christmas market there, but it was quite small.

You call this "very little pay-off"?

I agree though that it's not worth traveling three hours one way just for the Christmas market. She mentioned her original plans of traveling via Luxembourg, in which case Trier would have made a lot more sense along her route.

Tim - thank you for the link! Rheinfelden sounds very neat... not sure if we'll make it this time, but definitely putting it on the list! I'm hopeful we'll be able to visit Basel, but we will just have to wait and see if any of the requirements to get in/out of each country changes.

I know that Trier is a bit further away, but we have a love of Roman ruins and are mostly going to see those - the market looks cute too, but we wanted to choose places that interested us outside of the markets. I'll look into a few of the other places mentioned as well.

We talked about Wurzburg and it's not off the list, but we're slightly leaning towards Köln.. and yes, I know it's totally in the wrong direction! We love train travel, so we aren't put off by the 2.5 hours from there to Stuttgart - just more scenery to see! This is our first international trip since Jan 2020, so we're giving ourselves more grace and downtime than usual. We've also never traveled much in the winter, so while we're taking the right clothes, we want to be in places that have plenty of museums and such to escape the possible cold and rain.

I know Rothenberg is easy to get to via train from Wurzburg, but our real interest lies in Nordlingen which isn't as easy by train (I'm a huge Willy Wonka nerd.. ha!). There are SO many places that we want to visit - never enough time or money!

You call this "very little pay-off"?

No, not Trier itself. I was thinking of the Christmas market as Jennifer's main goal, I suppose, which would not justify the detour. Trier's definitely worth seeing, as long as Ancient Rome is on your list of musts.

"I know Rothenberg is easy to get to via train from Wurzburg, but our real interest lies in Nordlingen which isn't as easy by train..."

Nördlingen is worthwhile on several grounds. I'm thinking most of a day would be adequate. If it's a must-see for you, it's a lot closer to Stuttgart than Trier (3.5 - 4.5 hours one way.) Nördlingen is less than 2.0 hours each way. Arrive before noon, stay for dinner, and you'll be back in Stuttgart before 9 pm.