With 14 hours, I'd rent a car and head to Rothenburg ob der Tauber:

Lovely old medieval walled city - I really like it.

It's about a two hour drive from FRA, though I would plan 2.5 hours to be safe. The A3 often has a lot of traffic.

http://maps.google.com/maps?f=d&sour...72&ie=UTF8&z=9