Joan and Darren, I lived in Germany four years and not too far from Frankfurt for two of those years. The Main Radweg is very easily accessible. My suggestion would be to assemble your bikes at the airport and then board the S-bahn to the Hauptbahnhof (main train station, about 10 mins). There are several hotels very near the train station and they would probably be accommodating. From the main trainstation you head due north about three blocks to the Main River, walk your bike down the stairs (ramp for that purpose!) mount up and pedal east (hang a right) and enjoy a very good bike trip. I have ridden the Mainradweg in its entirety so please feel free to email me at firstname.lastname@example.org
with any questions. I have some good materials in German as well that I would be willing to share.
Highly recommend Aschaffenburg's Schloss Johannisburg, downtown Wurzburg, and Miltenburg (a very charming little semi-walled town in a huge bend in the Main). I also recommend Dionysius, perhaps the best Greek food in Germany right beside the Main River in Aschaffenburg.
Have a great time on your trip!