Frankfurt does have its pretty corners though, and it has many great museums. Plus, it is way more cosmopolitan than Mainz or Wiesbaden. Downtown is also very easy to get to from the airport (15 to 20 mins with frequent service on the S-Bahn - every 10 minutes or so). You could get off at 'Hauptwache' ('Main Guard House' - the 5th stop from the airport), and walk along the Zeil (main shopping street). Then turn right to get to the main square with city hall, and the cathedral not far away. The Schirn gallery of art is right there too. Then you could retrace your steps to the Zeil, and continue on to 'Konstablerwache' (where you can hop on the train back to the airport).