I am guessing here... won't your planned river cruise take you to Cologne and to other Rhine River towns?
I don't know this part of Germany at all and mostly want to find an
interesting place to stop on the way back to Amsterdam.
Guessing here again... your cruise will NOT include the MOSEL RIVER VALLEY, a place that most forum members here seem to find very interesting. The Mosel is not just picturesque (as everyone will tell you) but also full of nice places to visit. This river is a Rhine tributary full of old-world wine towns, castles, and hiking/biking options; access to these places by train is easy from the Rhine River town of KOBLENZ (to the south of Cologne) where the two rivers meet.
Mosel River Map
Burg Eltz is a much-loved destination on this forum. This intact medieval castle is a short bus ride from the Mosel town of Hatzenport - or a nice one-hour walk from the town of Moselkern.
Mosel villages
Beilstein (not on the list above) is an easy 1-hr river cruise from Cochem.
Cochem
You could conveniently stay in either Cochem or Hatzenport for your visit.
I would leave Switzerland as early as possible on this travel day so that you get in a few hours' sightseeing time that afternoon/evening.
When you finally leave the Mosel for Amsterdam, you will travel through Cologne - so you can stop over there if you wish.
I like both Mainz and Heidelberg - Mainz more than Heidelberg - but for your short time I think the Mosel and Burg Eltz will be more impressive and memorable.