score:0
Have you considered simply going directly to Munich and spending the whole weekend there? It's a fantastic city. Just take the train to Munich, no need to rent a car, and enjoy the city. Be sure to choose (any) hotel in the old town, or you miss the whole point.
If you do for some reason want to see "the Black Forest" you realistically need a car. (It's an "area", rather than one destination.) You can do that easily on Friday night; depending on traffic you will get there late Friday night (leave DΓΌsseldorf as early as possible to enjoy yourselves more). Yes, you can trivially drive to Munich airport Sunday from Schwarzwald area. So it's a simple 2-day car rental, one way rental from Dusseldorf to Munich airport.
I strongly recommend Hotel Diana in the Black Forest. Take all meals there!
Upvote:4
Can be done but it's not clear if it's worth it.