I'm surprised no one has mentioned Bamberg. It is a beautiful little city with many pretty little trails along the river. If memory serves, didn't the whole town get the UNESCO designation?

It is also a great place to eat some Franconian food and drink the Rauschbier. Bamberg should be a fairly easy train ride from Frankfurt.

If the OP is interested in wine tourism, there is also the not-quite-as-popular as the Rhein region around Wuerzberg.