Public transit friendly day trips from Frankfurt

Limburg an der Lahn, Idstein

Have you considered Miltenberg and / or Aschaffenburg? If not, google them and see if they hold any interest.

Easy trips from Frankfurt with the train - Limburg, Marburg, Idstein, Büdingen, Seligenstadt, Eltville and Eberbach Monastery, Bad Homburg with the near-by Saalburg Roman Fort and the Hessen Park Open Air museum.

Visit the 9 Nov. Initiative in Frankfurt, which is in a WW2 bunker. Something unique.

It sounds to me like you might have missed out on the Bergstrasse old-world towns which line up neatly on the route just south of Frankfurt between Darmstadt and Heidelberg. Wine is big here. Heppenheim is a fine little town, maybe the most popular of the bunch, which forum member Tom describes well:

In my mind, Heppenheim is a quintessential southern (non-Bavarian)
German town, in all the full scope that implies. Not just a preserved
historic core that displays all architectural motifs that people call
"storybook" or "fairy tale", but nieghborhoods that show the full
spectrum of German history, including Classicism, Grunderzeit,
Jugenstil and modern. Heppenheim isn't a hollowed-out tourist
funhouse, but much more typical of Germany in that the history sits
side-by-side with all the modern infrastructure that a town needs to
meet the economic needs of its people. So, although you'll see the
historic quarter with all of its restaurants, the castle looking down
on the town from a low mountain, and vineyards lining the lower
slopes, you'll also see modern high-rises and factories in the outer
areas of the town.

There are direct trains between Frankfurt Hbf (main station) and this collection of towns; the link below spells out the route and provides a glimpse of what you can see/do there. It's roughly 35 minutes to Heppenheim, less to the closer towns (Zwingenberg, Bensheim.)

https://diebergstrasse.de/wp-content/uploads/2025/04/Holiday-Route-2024.pdf

Something I regret missing was the Chagall windows in Mainz. Not off the beaten path, but an easy day trip from Frankfurt. https://www.atlasobscura.com/places/the-chagall-windows
If you fancy a spa day, Wiesbaden is on the S-Bahn.

following along from Ms Jo's suggestion of Bad Homburg, I'd second that and suggest all of the Taunus mountains in that area, to include Oberursel which has a small but lovely altstadt within a short walk of both an u-bahn and also an s-bahn station. That area, what isn't covered by s-bahn and u-bahn has good bus coverage, and while very close to Frankfurt it has a completely different feel.

How about the Frankfurt city park in the woods somewhat out of town, on the u-bahn or a tram, I forget, I think a tram, very nice to get away from the city feel.

I suggest Marburg and Bad Ems and for a place to see evidence of WW2 I suggest Koblenz. That museum will pique your interest as it pertains to WW2 too.

Thank you everyone! Great suggestions!

There's a place in the Schwarzwald, in Bad Württemberg, where I've spent over two weeks since 2000, that I love. It's called Bad Herrenalb. It's about 1H40min from Frankfurt (an hour by long distance train to Karlsruhe, then another 40 minutes by streetcar up into the Black Forest.

It's a cute little Black Forest town with lots of hiking possibilities and a spa, which if you're not too shy, is nice (the best parts, saunaland, etc., are "kleiderfrei").