I was about to recommend you the historic archictecture and museums of art and history.

You can visit (no museums okay!)
Bockenheim Strasse
Many of Frankfurt's liveliest bars, cafés and restaurants are in Bockenheim (sometimes called Pig-Out Alley), a working-class district that has evolved into an alternative scene center, heavily populated by students and bohemians. Food shops offer cheeses, fish (smoked or fresh), and local favorites such as Frankfurter sausages.
Brotfabrik
Brotfabrik is one of the city's most original performance venues, featuring live and disco music, heavy on salsa, African and Asian sounds. A restaurant and café are on site.
Bachmannstrasse. Call (069) 978-455 for more information.
Palmengarten Und Botanischer Garten
Tropical and semitropical greenhouses contains flora, including orchids, and palms. The surrounding park has a lake with rowboat rentals. Between the Palmengarten and the adjoining Grüneburgpark, the botanical gardens have an assortment of wild, ornamental, and rare plants. Flower shows and exhibitions take place throughout the year.
Siesmayerstr. 63, Frankfurt. Call (069) 2123-3939 for more information.
However I think you should visit
Goethehausthough it is a historical Museum.
The Goethehaus is the birth home of Johann Wolfgang von Goethe, one of the greatest figures in Western literature.
Grosser Hirschgraben 23-25, Frankfurt. Call (069)138-800 for more information.