Other excursions should be an option when you get on the ship. Also is need any info about any ports that Rick Steve's doesn't cover, just ask, I may be able to help and also Not a Mermaid.
Please keep us posted when on the Main River. I would like to know if you stop in Karlstadt in the morning to drop off people going to Rothenburg. If you do, a short stroll in Karlstadt (a walled town) is nice.
Have fun!