Heidelberg or a Rothenburgh
 

Heidelberg is much closer and a real city - castle of your dreams - if not rain hikes above town - legendary student pubs - one of few German cities not blitzed in war - some say because Allies wanted to use it after was as HQ and that happened.

Rothenburg takes a lot longer and is basically a museum town full of tourists - which is OK but two very different towns. I'd chose Heidelberg but others may prefer Rothenburg.

Threads like this are simply depressing (regardless of the misspelling and lack of basic information like mode of transport or interests). Do people do even the most basic research and read up on the places near Frankfurt? Instead of looking at the basics like Marburg, Limburg Gelnhausen plus Büdingen, Seligenstadt and Wetzlar always the same two, three cliche destinations. Rothenburg is certainly not a day trip from Frankfurt by train and barely by car. Why would you go so far when you have so many options more nearby? And Heidelberg is another eternal cliche though this post 1689 town has not that much to recommend it beyond a castle ruin.

https://www.google.com/search?q=heid...iw=944&bih=539

Heidelberg to me is one of the nicer German towns I've been to but locals' wants are different from American tourists often.

I think Lubsitch makes a good point - Limburg for instance is about 35 mins from Frankfurt by train and is a really lovely town.

But if it must be Rothenburg or Heidelberg - then Heidleberg is the only one that makes sense (and I'm sure your google skills are on par with Pal's)

Yes and actually going to a Marburg or Limburg may be better overall with the lack of American tour bus hoards, etc.

Marburg was a really great university town with neat castle or church or both on a hill in town. But people do like to go places they've heard of.

Worms is very interesting with the Jewish cemetery and cathedral. I enjoyed spending the night at Wertheim am Main. Easy by train or car. So many charming places and great scenery in the area.

Speyer is another neat town near Frankfurt - but they want to get out of the rain forecast I guess- personally I'd stick to original plans rain or shine as rain doesn't usually last all day and boats have under cover areas.

What are you looking for?
From ease of trip, Heidelberg is simpler than Rothenburg. Rothenburg can also be done as a day trip from Frankfurt. Depending on how early you care to leave Frankfurt and return late, you can get about 6 hours in Rothenburg even by train. Whether it is something you care to do depends on your objective. The worst thing I did on my first trip to Europe was trusting other's assessment of what were worth doing. I have done a trip where it took 5 hours one way to visit a place for one hour. A simple ratio person would not do this kind of trip. However, it was a life-time goal. It was one of the most memorable magical trip to Germany that would have been shot down by naysayers.
The weather on the Rhine can be tricky. I was traveling with first timers and was also deciding whether to do the Rhine due to rain. I had several options lined up, but found out early in the morning that the weather was good enough to do the Rhine. They enjoyed taking the gondola over the Rüdesheim vineyards and watching the ruined castles along the river and visiting one of them.