I don't know why you think that Frankfurt is not the place to be. There are soooo many museums here to visit, so much history to be found, medieval churches to see, as well as lots of Jewish cultural heritage. I can list you dozens of things to see and do here that would take you a week to see all of them.
You do know that Frankfurt was an Imperial city don't you? That the Holy Roman Emperors were elected here beginning in 1152 and crowned here since 1562? That this was the home of the Passion Plays? The 1st German Parliament? That one of the oldest churches in Germany is here, built in 850? That one of our Romanesque churches has the only "hanging vault" in existence? Did you know that Frankfurt had one of the oldest and largest Jewish ghettos, lasting from 1462-1810? Or that Anne Frank was born here and that her name is also included on the Jewish Holocaust Memorial Wall as well as 12,000 other names?
Why this stuff is ignored by the guide books, I just don't know. Personally, I find the sense of ancient history fascinating here. I love the Farmer street markets in all the old, turn of the century neighborhoods, with their cobblestone streets, the Klein Markt Halle with the butchers, bakers and produce on display, our old guard towers, I even love our skyscrapers. Frankfurt is so beautiful at night, or walking down the river, or hiking through our city forest.
So that you do not have to go so far away as Koblenz, you might consider a visit to the town of Bad Homburg, which has the Saalburg, the only reconstructed Roman fort in existence.