one day Rhine River Cruise

I am not sure what is the 'area of the 30 castles'; the mist scenic stretch of the Rhine for a cruise is between Bingen and St. Goar (and it stays interesting a bit further to Boppard). The Rhine flows through a relatively shallow but steep gorge, with cute villages and lots of castles on both banks.

Two companies operate cruise boats: KD and Bingen-Rüdesheimer. It is easy to stop the cruise whenever you want because rail services run on both banks of the river.

Bingen is not too hard to reach from Mannheim, but it still takes something like 1.5-2 hours. Why Mannheim? If it is not work or family related, I would forget about Mannheim. Nearby Heidelberg would be a better base (it is farther from the Rhine Gorge but many years ago, in a single day I easily travelled from Heidelberg to Bingen, dropped bags at a locker in Mainz on the way, cruised from Bingen to Boppard, and took the train back to my next stop, Frankfurt), or Mainz if you want to be closer to the Rhine gorge.

Take a look at the timetables for the two companies mentioned above. They can be kind of confusing. You’ll find that traveling by boat with the current (Mainz to Koblenz) is much faster than traveling the opposite direction. You can optimize your time by traveling by train to your desired boat departure point, taking the boat downstream, stopping at some of the cute towns (like Bacharach), continuing by boat to Boppard, having dinner by the river, then traveling by train back to wherever you are staying.

We stayed in Boppard, took the train to St. Goar, explored the fortress, train to Bacharach, explored, then boat from Bacharach to Boppard. Very nice day. I think I would get bored spending the whole time on the boat. The scenery is pretty, but you really should explore at least one town.