You may wish to consider the Frankfurt Card for the first two days for travel and other benefits. And I second the others here that the S-Bahn is just fine for travel to the city.:



Where can I buy the Frankfurt Card?

The "Frankfurt Card" is available at the following tourist information centres in Frankfurt am Main:

Tourist Information Hauptbahnhof [main railway station]: Monday to Friday from 8.00 hrs. to 21.00 hrs., Saturday, Sunday, public holidays from 9.00 hrs. to 18.00 hrs.

Tourist Information Römer, Römerberg 27: Monday to Friday from 9.30 hrs. to 17.30 hrs., Saturday, Sunday, public holidays from 10.00 hrs. to 16.00 hrs.

VERKEHRSINSEL Mobility Centre, An der Hauptwache: Monday to Friday from 9.00 hrs. to 20.00 hrs., Saturday from 9.30 hrs. to 18.00 hrs.

“Reiseland” DB travel agency at the main railway station: Monday to Friday from 8.00 hrs. to 19.30 hrs., Saturday from 8.00 hrs. to 16.00 hrs.

Frankfurt / Main Airport Fraport AG, hotel reservations/travel agency, Terminal 1, Arrivals Hall B: Daily from 7.00 hrs. to 22.00 hrs.
See http://www.rmv.de/coremedia/generato...tCard__en.html

Note that it is not sold at the train station, so buy it before you go there.