Andreas is truly more of a resident Frankfurt expert than I am, so hopefully he will add to the list of suggestions. Anyway, here are the top things I suggest seeing in Frankfurt:
Jewish Holocaust Memorial Wall - now with over 12,000 names on it, * Medieval Jewish Cemetery - one of Germanys largest and oldest, * Judengasse Museum & the Jewish Museum
St. Leonhards - built in 1219, * Alte Nikolai - built in 1290, * St. Justinus in Höchst - built in 850, * Deutsche Ordens Church - built in 1309 (these churches are all original, not rebuilt other than some roofing) * Kaiserdom - Imperial cathedral where the Holy Roman Emperor elections and coronations were held
*Römer and Römerberg, * Klein Markt Halle - indoor produce martket, * Eschenheimer Turm - guard tower built in 1425, * Alte Oper, one of Europes prettiest, * Karmeliter Cloister - largest wall paintings N. of the Alps, painted 1513-1519 * the Paulskirche, home of Germanys 1st elected parliament in 1848 - JFK also spoke here in 1963, * Main Tower, * Goethes birth house, * Palmengarten
Städel Art Museum, * Schirn Art Museum, * Archeology Museum, * Museum of Modern Art, * Communication Museum, * Kunsthandwerk Museum, * Liebieg Haus Sculpture Museum, * Icon Museum, * Bible Museum, * Leather Musesum, * Dialog Museum in the Dark, * Geld Museum, * Kinder Museum * Senckenberg Museum
Neighborhoods of Bornheim, Sachsenhausen, Höchst, & Westend
Depending on when you will be here, any number of festivals may be on.