Looks alright to me.
Nuremberg: not sure which of the many offerings here interest you. Rick Steves tends to emphasize the WW II era stuff, but there's much, much more to see and do here. I could easily spend 2 days there without bothering with the Nazi Doc Center, the Nuremberg Trials, or the NSDP rally grounds. So with a day trip, I'd say 3 nights minimum.
https://tourismus.nuernberg.de/en/discover/places-of-interest/
https://tourismus.nuernberg.de/en/discover/museums/
Most people on this forum will likely send you to Rothenburg instead of Bamberg, but I think that's because most people on THIS forum follow Rick's advice - not that Rick doesn't like Bamberg, but truth is, he doesn't even mention it, or the fact that it's one of Germany's best-preserved medieval cities (with UNESCO World Heritage status in fact.) So most of Rick's followers have never been there - only to Rothenburg. But Bamberg is the sort of place I prefer - a living, breathing city populated by Germans from all walks of life, a university town with lots to see and do and plenty of history to uncover, and some terrific brewery-restaurants with a distinct Franconian flair; it is possibly the one place within the cultural region of Franconia that shouts "Franconia" the loudest. Rothenburg OTOH promotes itself heavily and has carved out a niche for itself that is best described as "for tourists only." It's a very attractive town; its very cutesy storybook appearance can be attributed to the post-war rebuild that took place after 40% of the town was destroyed by allied bombs. Today it has an old-town-turned-theme-park vibe. The small Germany population that lives there is employed in the tourism industry - catering to the tourist with all the usual touristy stuff. On an average day, tour buses, cars and trains bring around 5,000+ visitors into town - if they didn't cater to all these folks, they'd be overwhelmed, I suppose.
So I guess your choice depends on what you are looking for. But you don't really have to choose. It is POSSIBLE to see both places with half a day in each if you feel energetic and if you have just a few specific things to see/do well in focus.
Some will surely say that you MUST spend a night in Rothenburg, the idea being to see it late at night or early in the morning when the tourist crowds have thinned. But there are some changes of train getting between Nuremberg and Rothenburg which are a little more doable without luggage. And Rothenburg is close enough to Nuremberg that those early/late options are possible by day trip as well. And... when you travel to Boppard, it will be simplest for you to make that trip from NUREMBERG rather than Rothenburg (which will again trigger additional changes of train with bags.)
1) Sample morning day trip to Rothenburg:
Nuremberg > Rothenburg 8:37 - 9:51
(3.5 hours in Rothenburg)
Rothenburg > Nuremberg 13:09 - 14:55
2) Sample morning day trip to Bamberg:
Nuremberg > Bamberg 8:40 - 9:20 (direct)
(4.2 hours in Bamberg)
Bamberg > Nuremberg 13:39 - 14:19 (direct)
(Going in the morning always allows you the flexibility to stay longer in town, should you feel compelled to do so.)
Tickets: The Tagesticket Plus (€25.50 for 2 adults) is a day pass that covers any/all regional trains all day long within the VGN zone. Both daytrips I mentioned are possible with this day pass. Note that if you buy the pass for Saturday, it is valid on Sunday too. On other days of the week, you will need a pass for each day.
Boppard: yes, the hotel should get you your Guest Card automatically, but request it if that doesn't happen.
