Two-night escapade from Frankfurt: Rothenburg o.d. Tauber or Middle Rhein?

down river follows the flow of the water down from the mountains to the sea. So you are going with the current.

Up river means going against the flow of the river, back up to the mountains in Switzerland.

Down river is much faster than up.

Towards the Netherlands is down river, towards Switzerland is up.

Easy to park your car, get a boat and return by train. Or park your car at where you will get off, take the train to the start and board a boat there. Even simpler than it sounds.

However, I also worry that two days and nights in Rothenburg may be a bit boring

Keep in mind that 2 nights really only works out to one full day. The first day and the day you leave will be partial days, so it's really a good amount of time for staying in Rothenburg. And if you haven't been there before, it's certainly worth a visit. I love it mostly because of the Tilman Riemenschneider wooden sculptures in St. Jakob's church, but it has a lot going for it.

You could also stop in Wurzburg on the way there or the way back, which is a lovely city known for its architecture and wines. I really wanted to visit there on my last trip but could not work it in. However, it's well worth a stop and it's right on the way to Rothenburg if you're driving.

There is much more to see and do on the Rhine... much more than just the cruise... and it's much closer to Frankfurt.

"What's confusing me is the whole mechanics of car, train, cruise, up and down etc. Feels like a lot of work for two nights."

The only work involved is getting a plan together. Here's one that would work.

Day 1:

• 8:53 > 10:04, Direct train from Frankfurt's main station to Rüdesheim. Drop bags at hotel.
• 11:00: Board KD cruise boat to Bacharach (ar. 12:00.) Walk the town, lunch, explore the hillside towers and enjoy the views on foot.

town map: https://www.informagiovani-italia.com/mappa_bacharach_am_rhein2.jpg

• 15:15: Board a 2nd boat to continue your cruise north past Oberwesel and the Loreley cliffs to St Goarshausen (16:00.)

Here's a shot of the legendary cliffs with St Goarshausen and Katz Castle just upriver:

https://i2.wp.com/herbert-piel.de/wp-44b92-content/uploads/2016/01/Loreleytal.jpg?fit=960%2C641

Options from St Goarshausen:
- Get off the boat, walk to train station, board 16:27 train back to R'heim; train ride takes 25 minutes.
- Return to R'heim by cruise boat... just stay on the same boat and you'll be back in R'heim 3 hours later at 19:00.

Day 2:

• A little morning time in R'heim (Chairlift ride to lookout and walking trails? A little touristy fun like this?)

• DIRECT train ride (48 minutes) to Braubach for Marksburg Castle... This one is a no-brainer IMO. Marksburg is the only intact, tourable castle from medieval times on the Rhine - and the 1-hr. tour is very good. Trains run this riverside route all day long... it's Germany's most scenic train route. Braubach has a stunning old-town zone and is easy to explore on foot. Walk up to the castle from town takes 25 minutes. Castle tours in English are typically at 1 pm and 4 pm. Braubach is a nice place for a meal too.

Braubach: 3 pages of photos: https://www.stadtbild-deutschland.org/forum/index.php?thread/7322-braubach-am-rhein-und-marksburg-galerie/
Marksburg Castle: https://www.deutschland-luftaufnahmen.de/wp-content/uploads/2020/10/Marksburg.jpg
Marksburg website: https://www.marksburg.de/en/circuit/#/
Old-town restaurants: https://farm5.staticflickr.com/4143/4751924826_98980aa49f_o.jpg

Return to Rüdesheim in the evening on the same train route that got you there. Maybe after some wine at the Weingarten on Braubach's riverfront:

https://www.klinge-gastro.de/weingarten/

Ticketing advice for boats and trains is easy to come by here if this interests you.

2 nights in Rothenburg is not too many we stayed 3 nights and had a lovely visit. Besides the beautiful alter Mardee mentions, there is an exceptional Night Watchman tour, a don’t miss, and the not icky at all crime and punishment museum. It is done and presented very well, not sensationalized.
Personally, I would save the Rhein for when I had a few more days.

I've been to both places three times, and would definitely choose the Rhein experience. Some of the towns on the river are as nice as Rothenburg, just without the walls.

Do what Russ suggests, his advice is spot on.

Footnote: There may be some train scheduling changes/disruption on the WEST bank railway during May due to construction; adjusted schedules are not yet in place. My suggestions intentionally have you using only the EAST bank railway for this reason.

Wow! I knew this forum would be the right place to ask, but this is beyond expectations. I'm going over each comment right now. And I'll make sure to update this with my choice and experience. Thank you so much!

This time of year go to Main Franken. Spargel und Wein.

You could also stop in Wurzburg on the way there or the way back, which is a lovely city known for its architecture...

Agree with Mardee.

The train trip from Frankfurt to Rothenburg and back is easy as can be evem though you have to change in Wurzburg and Steinach.

Another possibility to explore is spending the time in Miltenberg. Google it and see what you think.