Help with itinerary

Bingen is a good starting point for a cruise.
Koblenz is not generally a good place to end it. You end up on the boat for 4 hours and the last 1 - 1.5 hours aren't all that great. I'd end it in St. Goar or Boppard if you want to see the best part. Save the time and money and move on to A'dam from one of those towns instead of Koblenz. If you end it in Koblenz, you will also be a long, long walk from the train station or will need a bus ride to it.

The cruise from Bingen to St. Goar takes about 1.5 hours and costs €15 each (includes the 20% discount you get for showing the train ticket you used to reach Bingen.) The cruise to Boppard will be about €5 more. (The cruise to Koblenz is about €37 each normal price, so about €30 with a discount.)

The other problem I see here is that it sounds like you aren't doing or seeing anything in the Rhine towns??? Nothing at all of Germany except this cruise?? You can tour the interiors of some of these castles you'll be cruising past, you know, and the towns themselves are adorable.

Tour Rheinfels in St. Goar: Rheinfels Castle video
Rheinfels information

If you stay overnight in St. Goar it's easy to visit Bacharach and Oberwesel, both just a few minutes to the south:
Bacharach
Oberwesel
Boppard is great too. Take the chairlift ride to the Gedeonseck lookout for a beer or whatever:
Gedeonseck

Also, you can cross over on the ferry from St. Goar and take a train (22 min.) to Braubach. Marksburg Castle is in pristine condition after 700+ years and offers a good tour:
Marksburg website

It would also be possible to see Marksburg when you leave St. Goar for A'dam; get off in Koblenz, put bags in a locker, and take a train from there to Braubach (10 min.)

thank you so much ......you answered all of our questions.......thanks for the advice about not going as far as Koblenz.......and we definitely plan to stop and see towns along the Rhine and your response has made it much easier to plan our trip.........thanks again