Blockquote
Are you trying to get into the Black Forest or are you happy with
near? Neither Stuttgart nor Baden-Baden is in the Black Forest, and
whilst Freiburg im Breisgau is close it isn't either. Do you want to
get out and walk or climb, or do you want to look out the window? How
many hours each day do you want to put into the trips? Do you also
want to see some of Switzerland? Which side of Zürich will you be on?
I'm not sure if going all the way into the Black Forest is necessary or just being close is good enough, but I suspect with our time constraint that "close enough" will likely be enough.
We'd love a short hike, and we do want to get out of the car and experience things up close and personal. As for length of time....we can leave as early as we need and get back as late as we want, although it really is just a day, and will be followed by yet another day trip the next day, unless we combine the two somehow.
Finally, we will be seeing quite a bit of Switzerland. we have several days in country and our friends/hosts will be coordinating our Swiss adventures, although I couldn't begin to tell you which side of Zurich we'll be on. I know that they live about 10 minutes from the airport, if that helps.