Hi Peter.
I've been to Frankfurt twice (both times to the motor show as my wife is a bit of a petrol-head). To be honest, it struck me as a city for business rather than a cultural venue. There are lots of tall buildings and a few old markets. The journey on the train via Cologne went through some nicer-looking towns.
The area around the main train station has some old, slightly seedy, streets and alleys so a bit of street shooting (ideally with a camera ;-)) is probably order of the day.
The fast food (sausage - Frankfurter) shops were excellent and there was a flea-market we visited that might give you some opportunities. This wasn't too far from the station but, unfortunately, I'm a bit sketchy on street names.
Hope you have a good time. Paul.