phatbeetle -- there's an S-bahn train that runs from the airport direct to the main train station in Frankfurt so it's easy access if you decide to go a day earlier.
I've been to Frankfurt once briefly, and don't know the city well but if you're looking for a nice day out you can try Mainz, which is also accesible by S-Bahn from the airport (I believe it's the same line, the S8, as into Frankfurt but in the opposite direction.)
Mainz is a small city on the Rhine with a mix of old and new, and a reasonable intact medieval quarter. It's also home to the Gutenberg Museum, i.e., Gutenberg Bible, and focuses on printing and the history of printing.
The city centre is pedestrianised, full of shops and is quite lively at least when I visted last at Christmas. It's not super pretty but I thought it had a nice atmosphere and charm.