There are about 20 museums that might have something of interest for you.
- Judengassse Museum
- Liebieg Haus (sculpture)
- Applied Arts
- Film Museum
- Architecture Museum
- Museum of Modern Art
- Icon Museum
- Communication Museum
- Leather Museum
- Bible House
Several medieval churches, the most famous being the Kaiserdom (1415) where you can also climb to the top of the steeple, but you might also like the Alte Nikolai (1290)or the Deutsche Orden (1309). If you have time and it is the weekend, then go to the Justinus church (850) in Hoechst. I highly recommend seeing the 150 meters long Joerg Ratgeb wall paintings in the Karmeliter Cloister (1513-1521). If you are interested in Jewish history, there are the 2 Jewish museums, as well as the Jewish Holocaust Memorial Wall, and the rest of the Jewish Ghetto Wall, built in 1180. The Alte Oper is a beautiful building built in 1881, as is the main train station, built in 1888. In fact the whole bahnhof area is filled with these gorgeous turn of the century buildings.
The residential nieghborhoods that surround the city center were also mostly built at the end of the 1800's. Here you will find lots of small cafe's, and interesting little stores, as opposed to the typical chain store downtown. Bornheim, Sachsenhausen, Westend, Bergen Enkheim, etc.
if you want quaint though, then visit the neighborhood of Hoechst. Filled with half-timbered houses, all set up on the city walls, along cobblestoned streets, you will be astonished that this is the Frankfurt you may have heard about. This also includes a schloss and the Bolongaro Palace, as well as the above mentioned Justinus church.
If you are a foodie, one of the many Farmers Markets that are held on almost a daily basis in various locations in the city might interest you, as well as a visit to the Klein Markt Halle, and indoor produce market.
A stroll along the river in May during the day or the night, is always interesting as well as beautiful.
Enjoy your time in Frankfurt, it offers more that what you might have heard.
Edited: 1:10 am, January 14, 2012