My wife and I are stopping for a day and night in Frankfurt in September. I know it's mostly a business and banking center, but what's fun there? Museum and restaurant recommendations?
Try Frankfurt on Foot walking tour: http://www.frankfurtonfoot.com We really enjoyed it.
There really wasn't a lot. The area around the Hauptbahnhofwas sketchy at best. There was the Frankfurt Goethe House The Römersberg & Paulskirche was nice. There really wasn't a lot there. Mainz is nearby, nicer with a lot to do. Roman Theater ruins, shipbuilding museum which was really good. Parts of the old town were amazing.
Mentioned in this answer:
I personally would visit the Goethehaus, spend an evening in a typical outdoor restaurant by the river, drink some local apple cider (typical regional drink) and try famous "Frankfurter Grüne Soße" made of many herbs with boiled eggs and potatoes.
Try google mapping in @Sachsenhausen area, there are plenty Italian, Spanish, German food there.