While I like museums, I found the best highlight of Frankfurt to be a walking tour that I took -- couple of hours. Frankfurt has a history going back many centuries, like to the Romans. And there are things like the cobblestone markers in the sidewalk for people who were killed by the Nazis. And a little of the story of how Frankfurt came to be the financial capital of Germany. The tour was offered in English though might be tight timewise as I think it started like 1pm or so. But this was pre-COVID, so availability and timing may well have changed.