To ? From Frankfurt

My recommendation would be the nearby Rhine Gorge and Mosel Valley. Easy to get to by train from the airport. September is grape harvest time and wine festivals are going. Take a day cruise through the Rhine Gorge. Visit some of the many castles, like Marksburg and Burg Eltz. Easy train transport back to Frankfurt airport.

Another vote for the Rhine. You can do as much or as little as you want. Boppard is a good base, with lovely hotels right on the river. Bacharach is a super cute town nearby. Take the train south from Boppard. Stop in St. Goar and explore the castle ruins. Continue to Bacharach by train. Explore for a while, then return to Boppard by boat. You’ll pass lots of castles along the way.

The Mosel area is lovely also, with lots of cute towns. Probably better to have a car in that area.

I would suggest the other-way towards Würzburg, then branch out and explore Mainfranken. Federweißer and Zwiebelplootz.

There is also quite a lot to see (much more than the Rick Steves Guidebook talks about) in Frankfurt itself and within short day trips. Plenty for 3 or 4 days.

I also like Franken: Würzburg, Bamberg and Rothenburg among others. There are many good choices near Frankfurt.

Thanks for all the suggestions. Great ideas to explore further

Another vote for the Rhine.