Having been on the Danube and the Douro, we (my husband and I) were keen to travel on another European river. The Rhine is a "busy" river so plenty to watch. And the scenery so varied - starting in the flat Netherlands, going past the Lorelei and the castles on steep hillsides, through narrow gorges, then a somewhat industrial area with factories, silos, chimneys etc, and then to wooded areas near the Black Forest and finally the hills and mountains of Switzerland.
At every stop we had an excursion - usually a walking tour - with (mostly) excellent guides, using the wonderful Quiet Boxes.
One hundred and fifty two passengers, so we got to know some people, sat with them at meals, shared drinks in the lounge; not really possible on the huge ocean liners.