Frankfurt Hahn Airport has an excellent bus service into the city of MAINZ
http://hahn.orn-onlin...busfahrplan_mainz.htm
which is closer to the airport than Frankfurt is. It's a very pleasant city, with some beautiful old buildings and a superb street market. There are also plenty of bars and restaurants. I visited Mainz (on day trips from Stansted) last year and enjoyed it so much that I went back twice more. (I would have gone again this year but Ryanair have changed the flight times, making a day trip impractical).
This page has links to lots of information about the city:
http://www.germany-to...er_tlstadt-id1138.htm
Here's the city's official site:
http://www.mainz.de/W...t/hpkr-5nkek8.en.html
(Navigation is a little odd with that site. You need to initially use the drop-down options under 'Tourism' but then notice the menu which will appear down the left-hand-side).
Mainz is also a superb base for exploring. Frankfurt is about 40 minutes away by train:
http://www.frankfurt....rt_eval01.c.124836.en
plus there are river excursions available (on the Rhine) and loads of interesting towns to explore.
Chris