Traveling by car from Frankfurt to Bodensee

The Bodensee is quite a large lake, surrounded as it is by three countries. Which part are you going to?

I'd go "through" Baden-Baden, but I wouldn't stop there. Pass through Baden-Baden on your way to Offenburg, then go from Offenburg via Hauslach on the Schwarzwaldbahn to Triberg and onto Donaueschingen, then on to Singen. You should be able to do the trip from Karlsruhe to Singen on one train, or take a train from Frankfurt to Offenburg and change there for Singen.

From Singen, you should be able to take a train to Konstanz or Friedrichshafen on Bodensee..

Good plan, Lee, if they had used the train. I believe that that there's a car in there, which cuts down on the scenic value.

Do you mean the route or the cities themselves? With both cases, I would say the route along A5 that skirts the Odenwald and Black Forest is quite a bit more scenic. Baden-Baden is certainly a prettier city than Stuttgart, but there's no reason to stop unless you're a Rick Steves completist. The route by Stuttgart also experiences some horrendous traffic.

The road along the Gutach follows the rail line, but using a car would cut down on the scenic value - at least for the driver. There are some steep drop-off. No place for watching the scenery while you drive.