Day trip from Frankfurt

I'll be in Frankfurt a week after you. I chose Marburg and Eltville for two day trips and will mostly spend them wandering around. Eltville has the advantage of being in the middle of the Rheingau wine producing region, and there's a wine tasting stand on the riverfront that rotates producers through weekly to offer tastings of their wines. There's also a 1000-year old abbey (deconsecrated) with its own vineyards. Marburg is a famous university town with lots of steps and alleys to explore upper and lower towns.

I chose these from long, detailed responses to other such queries posted by Ms. Jo and Russ from Paradise. They're both regional experts who are generous with their advice. They often post not only their opinions, but links to videos and other information that gave me many happy browsing/planning hours.

Here are titles of a few of the threads I bookmarked, just put them in the search box. If you do some searches on Frankfurt, you'll find many similar queries.
Jo Travels Thru Germany
3 Days in Frankfurt Germany
Frankfurt 10-26 Nov

I would take the train (under 2 hours - one way) to Bonn on the Rhein River. The birthplace of Beethoven, art museums, 2 castles, a marketplace, Chancellor's residence. lots of culture and musical events. https://www.thecrazytourist.com/15-best-things-bonn-germany/

Day trips from Frankfurt that I have enjoyed: Büdingen, Eltville and Eberbach Monasteary, Limburg, Marburg, Gelnhausen, Idstein, Seligenstadt, Bad Hombug with the Saalburg Roman Fort and Hessen Park Open Air museum, Mainz, the Rhine Gorge, Worms, Speyer, Heidleberg.

Do leave a day for exploring Frankfurt as there is a lot to see here, including neighborhoods like Höchst or Sachsenhausen.

Thanks to all for your suggestions, will spend some time researching each of these.