Suggest you take the S-Bahn to Mainz instead - 20 min. or so - for a more pleasant outing.

Catch the S-Bahn at the FRA Regionalbahnhof (there's a second station there for long-distance trains as well.) Get off at the "Mainz Römisches Theater" station, not the main station; check out the Roman theater excavation on the far platform. Walk to the Rhein, to the pretty city park with views of the river (Stadtpark) and/or into the old town from there (along Augustinerstrasse toward the Dom - this is the old town pedestrian zone.) You'll find a lot of bakeries and coutdoor cafes in this area. There's also a nice Biergarten in the city park at the Hotel Favorite.

What weekday are you traveling? Maybe you'd want to visit a neighborhood market and some downtown Frankfurt historic sights.

In just 20 minutes the airport trains take you into the heart of downtown Frankfurt (sight #18 on the pdf document).

pdf-Document: frankfurt.de/sixcms/media.php/738/frankfurt_…