Black Forest

Other than the spa, to me, there isn’t much else to do. We had 3 nights/two days there. On one of the days we drove, then walked to Geroldsauer Wasserfall. It was a pleasant walk. At the end of the trail we stopped and had a drink at the restaurant.

We only spent one night in Gengenbach, arriving on a Sunday when some event was taking place. The town isn’t very large and I can’t imagine spending 5 days there.

Triberg was nice, but other than the waterfall there’s little else to the town.

To drive in Strasbourg the rental will need the French emissions sticker that your rental probably won’t have.

Addressing Jaimeelsabio's concern about your rental car: follow the RS guidebook to use the Park & Relais to enter Strasbourg. You park at a suburban lot and ride the tram into Strasbourg at a very reasonable cost and low stress.

There is other stuff to do in B-B besides the spa. and you will have a car so you can make tripsnto neighboring places. .3 days in Gengenbach ? No problem. I’ve spent a couple of weeks total in my 3 visits and have another 5 days this year.. Again, with a car you have many places to. visit

Good that you asked. I'd skip Baden-Baden. There's a very nice area to explore in between Kissleg and Baden Baden, much closer to Kissleg than Baden-Baden is - The Swabian Jura, which includes many Danube River towns, the Swabian Alb mountains, and many other nice places. Its western edge butts up against the Black Forest, so it's quite a convenient place for you. You won't find much if anything in Rick Steves on this region - I suspect he has never explored it. Places for you to look into independently:

Tuebingen
Sigmaringen
Burg Hohenzollern
Wasserburg Glatt (near Sulz)
Herrenberg
UNESCO World Heritage caves: https://whc.unesco.org/en/list/1527/
Hiking in the Swabian Alb:
https://www.komoot.com/collection/1985140/hike-germanys-low-mountains-far-reaching-views-in-the-swabian-alb

We recently spent a couple of splurgey nights at Karls Hotel in Sigmaringen - on the Danube with a fine view of Sigmaringen Castle and an excellent breakfast.

When you pick up your friend, do the pick up in Kehl Germany. Your friend takes a train to the German border station in Kehl (cheap local ticket from Strasbourg.) Service to Kehl is frequent and there are no concerns for you about driving into Strasbourg, parking, etc.

I agree with BB. There are lots of opportunities for day trips in the Black Forest. I spent four nights in Gengenbach and could easily have stayed longer. I will say that one of them was a day trip to Strasbourg, but there are plenty of other places I could have gone. And Gengenbach itself deserves a full day.

You could also take a trip one day on the Black Forest Railroad and stop at some of the really charming tiny villages along the way. (free with the KONUS card). Try this website for some ideas. https://www.black-forest-travel.com/places-of-interest/black-forest-railway.html

If I was to choose over two days in Baden Baden over two more days by the Bodensee there would be no contest. Kisslegg is closer to Konstanz and Konstanz is a superior destination. The city is more attractive, and there's much more to do. Mainau, Rhine Falls, Meersburg, Lindau, the cable car into the Alps at Dornbirn, Zeppelin rides over the lake, ferry rides across it, and all those neat little places on the lake shore.

The drive from Konstanz to Strasbourg is easy, and cuts right through the Black Forest, so it's scenic (although you should pay attention to the road.) The drive from Strasbourg to Baden Baden is nothing special.

If it came right down to it I'd rather stay in Freiburg or even Triberg over Baden Baden.

Baden Baden is not the only Therme in B-W. I much prefer one just south of Freiburg im Breisgau.

There are "Bad" (bath) towns all over Germany

Why not do Freiburg instead of Baden-Baden? You still have access to two thermal baths (the one in Bad Krotzingen and the one in Freiburg proper, there may be more), the old town is a Medieval/Renaissance wonder, and you have so many hiking opportunities at your doorstep, even if you don't have a car. It really is the Boulder, CO of Germany.

Never been to Gengenbach or Baden Baden. I know it is touristic, but a walk around the Titisee. Take a lift up to the top of the Feldberg and walk around. We stayed here once and they have considerably upgraded since: https://www.saigerhoeh.de/en/ With the car you can tour and see the country side. Years ago we found/plotted a route of the best waterfalls in the Black Forest. That was nice and took us to some very nice picturesque less touristic areas. I have a chainsaw carved wooden Fliegenpils in the hall from a home shop near one. Get some honey Schnaps.
https://www.schwarzwaldportal.com/en/_black-forest-waterfalls.html