To DoRent a cab - and drive along river Rhine. The valley offers a beautiful view with vine stocks running up and down the round hills. Go to Eltville, a small picturesque old german village and look at all of the cute houses.
Visit a vineyard - and try the Riesling wines of German. Many of the vineyards arrrange vine tastings in september. You can also go to the vinotek of the vineyard for some guided tasting. My two favorites are Weingut Robert Weil situated in an astonishing little village with a fanastic view. Combining an old german castle with a more modern touch building housing people when they have the vine tastings which attracts a lot of people with big and expensive cars from the neighborhood. My other favourite is Weingut Künstler with less impressive surroundings but with amazing riesling wines!
Visit a castle! It is a fine line between castles and vineyards, but you can never get enough of both. Schloss Johannisberg is beautiful and it is possible to wonder around the manors.
Take advantage of the termal springs in Wiesbaden and go to an all german spa. The extraordinary spa built on the former site of an old Roman sauna offers different pools and saunas. This year it is celebrating 100 years! Why dont you take your loved and do the treatment "Relax like jupiter and juno"? And just so you don't get shocked - no bathsuites allowed....