Ask for the ticket office of the Primus line, which you will find at the river side and buy yourself a ticket for a daily tour.
Get on the steamer and enjoy a breakfast, while the scenery passes by in slow motion. Here and there you will view a vineyard. Softly, old fashioned, traditional music will tickle your ears. As soon as your boat enters the Rhine river you will see vineyards by the dozen, where the famous Riesling grapes are cultivated, with castles imbedded and ruins high on the mountains on either side of the river. Get off at Rüdesheim, forget to be high browed, become a hipster and join the crowd. Later in the afternoon, after half a bottle of wine or so, you will be back on the boat to safely return to Frankfurt.