Go see Mainz. check out the Gutenberg Museum, the Old Town area. It takes 30 minutes to get there from FRA and the train service is very frequent. You can probably get a day room in some hotel in Mainz (I stayed at the Hilton close to the Hbf. It is a decent hotel), take a nap, and then amble around town.
If you are going to be there between 11/27 and 12/23, check out the christmas markets. It is fantastic for christmas shopping.
Enjoy!