The above locations suggested for Switzerland OR just keep going and go to Salzburg Austria. Very pretty town, but Austria is less expensive (admittedly I was there "a century ago"). It was funny we had come from Switzerland and others had come from Czech. We were saying how inexpensive Austria was and they were saying the opposite. We didn't want to tell them about Switzerland.
Since you're leaving from Frankfurt another idea is either hit the Rhine valley or there are also some towns around Frankfurt you could go to like Mainz, Heidelberg, and Rothenberg. I also read about towns like Trier and others around there which looked really interesting.
__________________
|