Another option would be to take a Middle Rhine cruise from Mainz each of the method to Koblenz (even though it will require about 6 hrs by boat). Nestled with the confluence from the Rhine and Moselle rivers, Koblenz is a reasonably metropolis best noted for its picturesque Previous City, https://gelegenheitenhub.de/digitale-erlebnisse-gestalten-einblicke-fuhrender-webdesigner-aus-koblenz/