If your flight takes to you Frankfurt (FRA) airport and you've got a few hours to spend, you could do the following to make use of it do to some sightseeing:

Go to the regional train station below Terminal 1, take regional train S8 (S-Bahn), exit at Taunusanlage station.
Look at nearby Alte Oper from outside,
walk through FressGass which starts right next to it,
walk through Goethestraße,
then walk to nearby MainTower for a nice view from above,
the only skyscraper allowing visitors on top of the building
(airport-like security screening when entering).
http://www.maintower.de/hlb/generator/S ... er.en.html

Afterwards walk to Paulskirche, then Dom/Römer with its historical buildings and tourist souvenir shops, and you have the essentials covered.
You could continue to the nearby river Main, cross the Eiserner Steg pedestrian bridge, and head to the right for lots of museums and Schweizer Straße with traditional applewine restaurant Wagner http://www.apfelwein-wagner.com/

Have fun, Kai