Hauptwache

... and sitting in the historic Café Hauptwache and watching the vibrant life of the multicultural city pass by is something you should definitely treat yourself to.

In the main guard soldiers used to defend the city. Later she was also a prison and has been a coffee house since 1903.

The main watch has been built three times in its history. The first building was demolished in 1728 due to dilapidation and a new, larger emerged.
In 1944, the main watch burned down after an air raid, but could be repaired.
When the subway was built in 1967, it was demolished and rebuilt a year later over the subway at a slightly different position.

Starting from 1709 stood in the immediate vicinity different Richtstätten: a gallows, a pillory, a Schaffott and further. Here a maid was beheaded in 1772 for killing her newborn, which was later immortalized in Goethe's "Faust".

More here:
de.wikipedia.org/wiki/Hauptwache_(Frankfurt_am_Main)

The historic Hauptwache building seems a little lost in the otherwise not particularly attractive square.

For fans of run-of-the-mill shopping, the Zeil is ok, but the days of Klever's specialty shops in the passages and side streets are virtually over and the former big names of the multi-story clothing stores have long since disappeared. What remains is the same as everywhere else in mostly ugly buildings.

If you want something a little more harmonious (but also more expensive), you should head towards Goethestrasse.