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