cities to just visit or stay in?

Heidelberg: "... would we be just as good to go there on our way to Rothenburg for part of a day."

Whether you stay there or stop over there, it's a major detour, and not worth the trouble IMHO. Not that it's a bad place, only that it doesn't have all that much on offer... and there are equally nice / better places (my opinion of course) on the way to Nuremberg and near Nuremberg, places that will not require the extra ground-travel time that Heidelberg would do.

One change I would make is to swap your Rothenburg time with Nuremberg. Do Nuremberg first, then Rothenburg.

From the Rhine to Nuremberg (or Rothenburg) your train must stop in Würzburg anyway, and W'burg is a great place to spend 4-6 hours or so and there are lockers at the station for your bags. Please have a closer look at what's on offer there if you have not already done so.

The other good option is to travel directly to Nuremberg, then make a side trip from there to Bamberg (a UNESCO World-Heritage old-world town, about 45 min. from Nuremberg by direct train) during your stay there.

Rhine: Boppard is the better travel base. You can get there from FRA by direct train in 1 hr. 15 minutes. Head straight there. Then use the free Guest Ticket you receive from your hotel hosts to visit Burg Eltz, Marksburg Castle, Cochem, Rüdesheim, and other places near Boppard which might be on your sightseeing list. Your free train rides will take you along both sides of the river. If you catch the train to Rüdesheim (or Bingen) you can cruise back to Boppard in a reasonable amount of time in the late afternoon.

When is this? Sorry if I missed that. Also, how many nights on the ground?
We were there this past May, and I missed the obvious about the river cruise- perhaps do it from Rudesheim to Boppard ( or reverse) on your way in or out of the area. Yes, you'd have luggage, but I don't see that as a problem on the river cruise. I think you can save a day this way.
You are going to some of my favorite places in Germany and Austria. The beauty of the Berchtesgaden area was breathtaking. I wished we'd stayed there instead of just the day trip, but I also loved Salzburg in the mornings and evenings.
I'd add a night to Salzburg and take one away from Munich, but that's just my bias.
We found Marksburg hard to get to, and wish we'd gone to Reinfels at St Goar instead .
Unless you have strong feelings, I'd skip the over- commercialized and very crowded Fussen castle. (There's much feedback here if you do a Search.)
Rothenburg is also magical to me.
Have a wonderful trip!

https://community.ricksteves.com/travel-forum/trip-reports/san-diego-to-munich-just-getting-there-part-1

Hopefully our recent trip report may offer some helpful info. Because of length limits, one can't post the entire report as one unit, so Hopefully you can find all 7 parts on the above thread.

If you have to make a huge detour then I wouldn't do. I would spend a night or two in Salzburg. Sure it is expensive but there are a lot of day trippers whereas at night is a little more manageable.

I would second Pat's rec to add to your stay in Salzburg and shorten the Munich trip. I loved Salzburg - I was there for 2 nights and had a wonderful time exploring the city and countryside. I really want to go back. Munich is fun and definitely worth a visit but I think you can hit the major highlights in a 2 night stay.

Heidelberg is great, but you might skip it for another day when you do the Black Forest and Strasbourg/Colmar.

Also, instead of Nuremberg, consider doing the Romantic Road from Rothenburg Ob Der Tauber down to Fussen/Garmish.

There are several Medieval walled cities that are similar to Rothenburg and largely undamaged in WWII. Nuremberg was pretty much leveled by allied bombing. What you see there today is largely new building built in the old style.

Salzburg has more to see than Berchtesgaden. The Eagle's Nest is awesome, but you can do that on a day trip.

https://www.romanticroadgermany.com
It's not too hard to see the reason for the popularity - despite the modern roots of the idea, the tour combines the historic cities of Würzburg and Augsburg with the three medieval walled towns of Rothenburg ob der Tauber, Dinkelsbühl and Nördlingen, and then finishes off with the tourist highlights of Neuschwanstein Castle and the Alps.

Where and when does your trip end? Are you circling back to Frankfurt to fly out?

Ok...a few more details.
End of July, beginning of August. 15 nights in total
Flying into Frankfurt and out of Munich.
Will have a rental car for part of the trip (picking up in Rhine area I think and dropping once we arrive to Munich)
Will be driving the Romantic Road....from Boppard will drive to Wurzburg to Rothenburg for 2 nights then to Nuremburg then backtracking to the Romantic Road to head towards Fussen.
I hear the naysayers about the Neuschwanstein Castle - but we really want to see it.
WWII history part of the interest of our trip. Will be doing a trip to Dachau from Munich.
From my brief look Salzburg hotels seem crazy expensive and with 15 nights in total on this vacation can't be paying what most seem to be...thus the questions on daytripping there etc...any advice on that helpful.

If you dont plan on spending time in Mainz, just go on to your first destination. Although you shouldn't be driving after a long flight. Have you looked at doing this mostly by train? It's much less stressful, and all the places you mention are easily reached by train. Cars add considerable cost in parking and time. The Romantic Road, is just a road that links some nice towns: in itself, it's not that exciting.

Agreeing that Heidelberg is a long detour. I would skip Cologne. One night in Rothenburg is enough - it's very small. And do it on the way from Nuremberg to Munich. The castles can be done on a day trip (even by a coach tour group) from Munich. Berchtesgaden as day trip from Salzburg, unless you have no interest in Salzburg.

We stayed in an RS fav in Salzburg, Boutique Hotel Am Dom, and I plugged in a few dates, and it seems sold out already. Yikes! I'll guess that's your problem, the reasonably priced hotels are already booked in Salzburg.
Berchestgaden is much more than Eagles Nest we found. The hikes, the beautiful lake cruises, and the nearby salt mine are all worthwhile.
Oh, how many in your party, and adults/ children ? The details help with the recommendations.
Again, have a great trip,

2 adults - celebrating our 40th Anniversary and my 60th Birthday.
I suspect we will train to Boppard from the airport and spend a couple of days taking advantage of the trains and cruise before picking up our car.
We like the idea of driving - we did it in France 5 years ago and like the independence and being able to stop when something looks interesting, however, we want to know where we are lying our head each night and a list of options and of course, some "must do's" . No interest in coach tours.
While we realize the trains are efficient, we want to limit our hauling of luggage in and out of stations etc to the beginning and end of our trips. Yes there will be times during that time that a car isn't really convenient but its the compromise we will have to make to have for when we want it. We have already eliminated the idea of picking up the car at Frankfurt airport because of the train efficiency of getting to and around the Rhine Valley.
Pat - I had also looked up that hotel from your trip post and saw it already unavailable. - Salzburg might be a challenge.
RS also suggests some hotels and B&B's outside the main area of Salzburg so maybe will have to try that if we decide to overnight there.

Congrats on the bday and anniversary!

I would stay in Heidelberg overnight. Get out some and enjoy the night life. I've never been to the Rhein in high season. Off season on the Rhein is terrible. Everything is closed. Be ready for super touristy in July.

Go to Salzburg, skip the Eagles Nest. Do the electric boat on the Konigsee. Beautiful mountains. For me much better than the Obersalzberg.

Edit: I drive. A car broadens your options.

"While we realize the trains are efficient, we want to limit our hauling of luggage in and out of stations etc to the beginning and end of our trips."

These days I travel exclusively by train in Germany. I'm not bothered by my luggage at all, and I credit Rick Steves and his very intelligent packing advice for this. DW packs as Rick advises as well. On recent trips she has experienced certain age-related mobility issues (we're both 70.) When boarding/deboarding some trains and buses, or when transiting between tracks (think stairs,) she needs both hands mostly free for safety. But it's not an issue for me to handle both our bags - as long as they are "Rick-packed" - for these brief moments. Otherwise, we each handle our own bag. We move a little more slowly both in stations and outside stations than we used to, but that's just how we are. It's got nothing to do with our luggage. It's the long walks in certain AIRPORTS that might bother us, but rail stations tend to be a snap by comparison.

So I tend to look at train stations as a travel BLESSING rather than a curse. I never have to search for or pay for parking. And when it's raining, I am not fumbling around with my umbrella in an attempt to stay dry as I leave the front seat, open the car trunk, and pull out my luggage with my third hand... and I'm not immediately running for shelter. I just step onto a dry platform, bag in hand, and then into a mostly warm/dry station lobby (where a hot cup of coffee often awaits me.)

Some additional logistical planning is helpful with trains as well... especially if you refuse Rick's packing strategies...

1.) Stay fairly near the station so that you don't have to walk for 20-30 minutes or more to your room or take a local bus.

2.) Use "hub and spoke" planning, like you are doing in Boppard, in other places. For example... make either Würzburg or Nuremberg the hub, and day trip to the other and to Rothenburg.

3.) Select hub destinations that can be reached with fewer changes of train. It's 1 train to a Salzburg hub from Munich, but 2 trains if Berchtesgaden is your hub.

4.) Don't duplicate experiences that cause additional journeys with luggage. Why go all the way to the Alps in Füssen, then to other Alps in Berchtesdaden, for example? Pick one alpine destination or the other, and stay a little longer.

5.) Turn a hub into a spoke. If you're staying in Munich at the end, don't make expensive Salzburg a hub; instead, add a night or 2 to Munich. Then do direct a day trip from there to Salzburg (1.75 hrs by direct train) and to Füssen or Mittenwald. I would do the Salzburg day trip on Sat or Sun (travel at any hour on the Bayern Ticket) and plan on a long day as there's lots to see in Salzburg.

I’d suggest staying in Berchestgaden or the immediate area and visit Salzburg from there.

We found Marksburg hard to get to, and wish we'd gone to Reinfels at
St Goar instead .

Boppard > Marksburg... pretty easy by train alone...

• train to Koblenz (change trains) + train to Braubach. Then scroll down on and use this map to get from the Bahnhof to the castle (#5) via the "walking trail."

Two additional ways to get from Boppard to Braubach...

• KD cruise boat 10:15 - 10:45; later, return by train via Koblenz... OR return by KD cruise 13:05 - 14:00... or KD cruise 18:55 - 19:45. From the cruise boat you will see the castle quite easily as you approach Braubach.

• Take the Boppard ferry from the riverfront downtown to dock on opposite riverbank; then take a 15-minute walk north to Filsen to catch the train into Braubach.

Marksburg stands head and shoulders above Rheinfels, IMHO. Marksburg is an intact medieval castle, never destroyed, with true medieval feel. Rheinfels is worthwhile too. But it's a set of ruins with a museum.

Braubach is a lovely little place for a stroll and a meal:

https://fotos.schloemp.eu/wp-content/gallery/braubach/002Braubach-Bauernschaenke-Eck-Fritz.JPG
https://farm5.staticflickr.com/4143/4751924826_98980aa49f_o.jpg

Russ, don't forget that 30 minute walk straight UP the hill to the castle. There were no local cars taking folks up on our day.
You all really tried to help us out on getting there and back, and for some reason getting back took forever, and we seemed to miss the connections, whatever, that's on us. It was also a very cold, windy, grey and blustery day - in May. And- tour was in German, with us trying to follow along with printed English. ( We did know this in advance.)
But this is all small stuff, the trip was great!

Having been to both the Marksburg and Burg Rheinfels more times than I can count, I would pick Burg Rheinfels as 10 times more interesting and more worth your time. The Marksburg is nice, but it IS tough to get to. If you have any kind of walking problems or the weather is icky, this is not a lot of fun walking up that hill, nor walking the steps. Their English speaking tour guides are sometimes very hard to understand. I do not know why they don't hire a better English speaker, but they haven't so far. The tour lasts barely 40 min. and then you are done. Time to leave. You spend a lot of time getting there and then that is it. Braubach does not really have anything there that is very interesting. There ARE other castles in Germany that have not been destroyed. This one is on the Rhein though and gets a lot of attention.
St. Goar and Burg Rheinfels are the complete opposite. You can spend as much time as you like walking around, they have tours of the extensive tunnels which are soooo impressive.. When you walk into the Great Hall, it will make you say WOW! I have spent hours wandering around and have never gotten tired of visiting this massive fortress. Lunch up on the ramparts is fun. The town has 2 gorgeous old churches, and a number of restaurants along the main street.

There is a lot to see in Heidelberg, not just the castle. The Apotheke museum is really interesting and it is free with your ticket from the funicular and castle entry. My favorite thing to see in the town is the Student Prison, but go into the Aula too while you are there. The church in the main square has a fascinating history, but make sure you go into the Jesuit church too. It is gorgeous.

You mentioned driving the Romantic Road starting in Wurzburg. You might want to start in Aschaffenburg, which some think of as the real start of "the road". Between there and Wurzburg, on the old road, is a place you might want to check out. It is a town called Messpellbrum. It has a small fairy tale castle complete with moat and has been in the same family for five or six hundred years. Google it and Aschaffenburg and see if they hold any interest for you. Also, since there is an interest in WWII, check out a site named thirdreichruins.com. It has many photos taken just after the war, and then photos of the same places in more recent times.

Mardee: I didn't mean to discount whatever difficulties you had on your visit. Just wanted to provide the transport options for this particular OP.

As Jo points out, Braubach has no majorly important sights beyond Marksburg Castle. Just a very pleasant little hamlet with some very nice half-timbered buildings from centuries ago. St. Goar's sightseeing options, except for Rheinfels, are equally meager, all things considered; the buildings there on the main drag through town aren't that much to look at. The views from St. Goar's waterfront across the Rhine, which include Burg Katz, however, are quite good.

But Marksburg castle is truly something. What you'll see there on your tour... judge for yourself: https://www.marksburg.de/en/circuit/#/

The hike up is fairly steep. Part of the medieval experience. You can take your time or you can take a taxi up. Note that castles - including Rheinfels - were almost universally built on steep cliffsides, obviously for defensive purposes. The walk up to Rheinfels from town, though possible along a sidewalk, is also quite steep. The alternative there in summer is a castle-shuttle-mobile for a few Euros, or a taxi. In fall, the shuttle is probably not available.

This family drove to Braubach, parked, then hiked up to Marksburg for the experience. You'll find video of the walk up between minutes 3 and 4.

https://www.youtube.com/watch?v=izJ383tD6XA

The husband (in German) says they spent about 15 minutes on the trail getting to the parking lot - and 5 minutes from there to the castle. He gets winded but still labels it doable. If the trail is mucky or icy, I would do the taxi.

I'd recommend seeing both castles, as they are very different experiences. St. Goar is just a 10-minute train ride from Boppard, so why not?