Southwest Germany with college kids

From Stuttgart you could go over to the Alsace area and see the super cute towns/villages there: Colmar, Eguisheim, Ribeauville to name a few. Get a little taste of France with a big German influence.

On your way from Stuttgart to Nürburgring : Mosel River and the Rhine between Bingen and Koblenz

Easier to reach from Stuttgart the Hockenheim Ring (1,5hrs from Benz Museum) if its just for driving (check link below)

Take a Renntaxi to drive the Ring

https://einmalige-erlebnisse.de/hockenheimring/renntaxi

Visit Speyer on the Rhine nearby with its Dom(Cathedral) the Sealife Aquarium and the Technikmuseum
as well as the Museum der Pfalz and The Historic Center

and a video of the Technik Museum

I'll second the suggestion of Speyer. The Technical Museum is great, although their sister museum in Sinsheim is better. But Speyer has a very historic cathedral, and it's one of the oldest settled places in all of Europe. It was a Celt settlement 4000 years ago. It's a town I just like to go to on a Saturday morning and just walk around.

Heidelberg is a University town, it has a great old city center, and a really good castle. It is a bit touristy, but there are a lot of international students there. It's one of the places I took my cousins two college age kids last month and they really enjoyed it. (Just tell them to explore on their own and meet someplace in 5-6 hours. They'll be okay.)

Another option would be, after you leave Stuttgart, to drive over to Trier. There you can explore the city that was second only to Rome until 500 AD and see the ruins they left. Then from there you can drive up the Moselle through all the pretty little towns and past more than a dozen castles. Plus it's wine country, and there's always something going on between Trier and Cochem.

Or you could go to Mainz, and from there up the Rhine to Koblenz.

Here's a link to local winefests: https://www.rheinhessen.de/weinfest/3?tx_sfdeskline_pi3%2525255Bseoid%2525255D=Mainzer-Weinsalon_Mainz5&dateFrom=01.01.2024&dateTo=31.12.2024&eCategoryIds%5B0%5D=23&eCategoryIds%5B1%5D=12

If you are driving from Füssen to Stuttgart and then Frankfort eventually, I would opt for areas around the Rhine.

But first, you could first stop in Tübingen, which is a bit south of Stuttgart, so just a slight detour from your route. It is a charming university town with a well-preserved medieval Altstadt, lots of half-timbered houses and really lovely views of the Neckar River (which you can take a punt-boat ride on). It also has Hohentübingen Castle, where you can get wonderful views of the town. And it has a really fun college vibe due to the presence of the University of Tübingen, which I think would appeal to your kids. It's well worth a visit. https://historicgermany.travel/historic-germany/tubingen/

After Stuttgart, just head up the Rhine River. You could take a river cruise along the Rhine (if you take the boat from Bingen up to Boppard, you will see castle ruins galore on either side). You could also make a visit to Burg Eltz, which is a fantastic castle that sits up in the mountains surrounded by incredible views. It dates back to the 12th century and has been owned by the same family for over 850 years. Visitors can take guided tours of the castle's interior, which includes really lovely furnished rooms with original artifacts, armory, and centuries-old artwork. Tours are both in German and English. There is a nice cafe on the grounds as well. I went here and absolutely loved my visit. It is easy to get to by car, also. Here is a link - check it out and you can see a photo of it. https://burg-eltz.de/en/homepage

Mosel Valley highly recommended. Gorgeous scenery around every turn. Burg Eltz is popular (Rick S says it's his favorite). We visited the ruined Burg Landshut near Bernkastel-Kues. Except for the top of the tower, it was free to visit with parking a short walk away. Beautiful views over the valley. And we had a great meal at the nearby Restaurant Waldschenke. Giant platter of meat. We stayed in a fantastic AirBnB in Enkirch, which was a 1.5-hour (ish) drive to Frankfurt Airport.

Do not reject the scenic middle-Rhine dayboats (Bingen to Boppard, roughly) if it seems out of your way, or the car seems like an impediment. The train will get you quickly back to the car. And it is incredibly scenic. (MANY posts on this board.) A car can get you to Burg Eltz easily, although apparently the forest walk from the train is very nice. I found the lived-in (occupied ... ) interior of the castle to be less attractive than the magnificent exterior.

If you page down in my trip report to Fachwerkstraße, you'll see some (non-Car) things we did south of Frankfurt.

https://community.ricksteves.com/travel-forum/trip-reports/baltic-germany-fachwerke-landesgartenschau

I was on a guided tour so can't help with logistics and we only went to the free Welt, but if you're starting in Munich, does your son know about the BMW Museum? https://www.bmw-welt.com/de/index.html

You can stay in a car themed hotel in Stuttgart https://motorworld.de/en/region-stuttgart/v8-hotel/
We didn't stay there, but my dad and husband were like two kids in a candy shop walking through Motor World https://motorworld.de/en/region-stuttgart/ and seeing all the different kinds of cars. People store their personal cars there. I remember seeing a blue Austin Healy, so not just German cars.

It is on an old airport, and when we were there in 2015, you could rent cars to drive on the runways. We did not do that. But it seemed like it would have been fun.

If your kids like theme parks, Europa-Park near Rust, Germany is worth a visit. https://www.europapark.de/en/park

My kids loved it. Perfect for older teens and adults. Considered by many to be the best park in Europe after Disneyland Paris, but much cheaper than Disneyland Paris. Lots of coasters. Each section has a different European country as the theme. Couple of hours south of Stuttgart. We are doing a trip there later this month and we are staying in Strasbourg. Flying into Stuttgart. Note your German rental car won't have the correct environmental sticker for France. Everyone parks their car in the free car park behind the train station in Kehl Germany just across the border. Then take the tram into Strasbourg. Line D stop is directly in front the the train station (part of the Strasbourg tram system).