Frankfurt is a bit "corporate"... It's the financial center of Germany. I have spent a few days in Frankfurt in summer 2001 and a total of 3 weeks in Germany.
You will certainly want to explore the Bavarian country ; don't waste your time on black forest, there is nothing to do there (except if you are a spa and natural hot spring lover). Take your car and head to Munich (nice city!) and go approx 125-150km south-south-west near the Austrian border to the Neuschwanstein castle. It's a mix of the Cinderella castle (in fact, the Walt Disney castle is based on Neuschwanstein (not a joke btw!)) and mountain background.
You will also want to visit the Dachau concentration camp near Munich. Really a must...
Last suggestion : take a weekend and head to Austria: Vienna obviously but also Salzburg (what a %#$%# nice city!! My favorite) and Innsbruck.
Food in Germany is horrible relatively speaking... Wine is ok but only the white (Gewerstraminer from the Rhine).