Planning now for next August for our 30th. My plan is to fly from DC area to Frankfurt. (should we stay here or head to first stop?) Stay in Heidelberg where my inlaws lived for a time. Then to Rothenburg, Wind our way down to Fussen and over to Munich and then hope over to Salzburg. wanting some ideas on how many nights in each town, how and where we can throw in a Rhine River cruise (not a week long one, just a day) and any other cities that are must do. We plan now to take the trains, but are open to driving if you think we can and should. We can do anywhere from 8-14 days and are open to going in reverse. But Heidelberg has been our dream and the Romantic Road and fairytale castles are a great theme to tie this celebration together. Suggestions, ideas, must see, recommended hotels or airbnbs or inns, tours, etc...thanks.
If you want to take the Rhine boat trip, take the train your first day to Bingen. Drop your bags at the hotel, then board the boat and take it to Boppard. Take the train back, stopping at any or all of the following - St. Goar, Oberwesel, Bacharach.
Train to Heidelberg. There is also a nice boat ride there along the Neckar.
If you really want to explore the RR, get a car. Spend at least one night in Rothenburg and one in Fussen, although 2 each would be better.
Drive to Salzburg and spend a couple of days in the area. The ice cave at Werfen is a unique experience. The salt mine tour is fun. There is one in Hallein near Salzburg as well as one in Hallstatt and Berteschgarten. Hallstatt is worth a day trip if you have a car, but I wouldn’t do it by public transportation.
Public transport along the Romantic Road is rather inconvenient. There are many lovely places between Würzburg and Füssen which are at a certain distance from the road. You will be make most of your journey if you do a bit of zigzagging.
To enumerate just a few places: Weikersheim, Bad Mergentheim, Creglingen, Schloss Schillingsfürst, Bad Windsheim, Öttingen, Ellwangen, Neresheim, Schwäbisch Hall, Harburg, Dillingen an der Donau, Augsburg, Landsberg am Lech, and the Ammersee.
OK. Heidelberg is across Germany from the Romantic road. It's south Frankfurt; Rothenberg odT is south and east. And the castles are either north along the Rhine or in southern Bavaria. (The touristy ones are.) If you want to see the river castles, you should do that before you head south. If you want to see the southern ones you can do that on the way to Austria.
I'd go to Heidelberg from Frankfurt, then take the Autobahn to Crailsheim and turn up to Rothenberg. Then go down to Nordlingen (a very nice little town most tourists miss), then thru Augsburg to Munich. From there you can take a tour and see the Bavarian castles, and then go to Innsbruck and Salzburg. Or you can just drive south from Munich to Chimsee and then to Salzberg. Either way is nice.
I always suggest three days in a city you want to see. You can't see everything, but if you pick out the stuff "you can't miss" you can probably do that. I've spent weeks in all the cities you're interested in, and still go back for stuff I missed.
"...any other cities that are must do..."
"Suggestions, ideas, must see..."
Suggestions really depend on what kind of trip you want. Most Americans - and other foreigners attracted to this part of Europe - get hooked by places that are highly "visible" in the English-language guidebooks and the travel shows. The biggest, shiniest lures tend to be Rothenburg, the Romantic Road, the Rhine Cruise, Munich, Salzburg, and the "fairytale castle" Neuschwanstein near Füssen. Heidelberg's castle is similar draw. (Add to that list anything associated with Walt Disney or Adolf Hitler.) So during high tourist season, these places become exceptionally loaded with tourists.
https://www.frankenfernsehen.tv/storage/thumbs/1200x630c/r:1475156718/95171.jpg
http://media1.s-nbcnews.com/j/msnbc/Components/Photo_StoryLevel/071210/071210_rothenburg_hmed_1p.grid-6x2.jpg
https://www.schoener-reisen.at/Bildergalerie/data/media/72/dsc02733.jpg
Since you seem hooked on the same places, and since you're celebrating 30 years of romance, I think it's wise to ask yourselves how many of your days of this special time you wish to spend in such places. There certainly are a LOT of alternatives. I'm sure you'll want to visit some places on your list. But I don't consider ANY of your chosen destinations as absolute "must-sees" (not that I really believe in must-sees anyway.) I think I'll just provide a few ALTERNATIVES for your consideration, places that offer romance but that are less well-known and will likely be less over-run.
Rhine cruise: Don't just cruise and leave. A couple of days are needed in the Rhine Valley, where you can explore medieval towns and castles in a more leisurely way...
Oberwesel has a lovely setting with a splurge castle-hotel (Auf Schönburg.) Walk the town wall trail from tower to tower. Just outside of town in the vineyards is the Günderodehaus, good spot for refreshments after a hike up. Nearby, Rheinfels Castle in St. Goar is a place you can wander through all day long if you like (Neuschwanstein gives you only a rushed, overcrowded 30-minute guided tour.) Boppard's old town, its riverfront area, and the Gedeonseck Terrace (take the chairlift ride to the clifftop) are nice spots as well.
Alternatives near Rothenburg:
Iphofen
Sommerhausen, Marktbreit, Ochsenfurt
Bad Windsheim's open-air museum
Fairytale alternative:
https://www.deutsche-maerchenstrasse.com/assets/prospekte/imagebroschuere_en.pdf
https://www.deutsche-maerchenstrasse.com/en/
If you want to see a town with all the important amenities of Rothenburg - Fachwerk buildings and an intact wall you can walk on - my best suggestion would be Nördlingen. It's kind of the little sister of Rothenburg, much less touristy, without a Christmas store or a Crime & Punishment museum, but otherwise quite enjoyable. It sits in an ancient meteorite crater; the Apollo astronauts trained on the crater walls near Nördlingen.
Only an hour outside of Frankfurt by train is Büdingen, a beautiful, medieval walled town that unlike Rothenburg, was never bombed. So, all buildings are original. Book a private tour with the TI and they will take you inside the massive stone walls and up on top of the huge towers. The tours are very reasonable and well worth the price for this unique experience. For some quirky information, Büdingen was a hotbed for witches trials and executed about 400 people in the space of less than 100 years.