yeah the ousider opinion about frankfurt is often quite bad. this has also todo with the frequent reports about the high crimerate in german news. however we know there are some bad places e.g the area around hauptbahnhof. but in general its not as bad as people think. the statistics factor in the immigration "crime" from the large airport. as you mentioned frankfurt is rather green compared to other large metropolitan areas and you can ride your bike or go on hikes pretty close to the city center. (frankfurt greenbelt or regionalpark rheinmain are good starting points)