Every Saturday from 8-14 is along the Main Riverside at "Museumsufer" Frankfurt's biggest flea market, where you can everything. It's very international flair from Turkey to Russia you find dealers and customers here! Plan 1-2 hours for a visit here!
To get straight into FFM life and culture, the best way to get around is by bike. Bike paths everywhere and the traffic totally respects cyclists.
On the North side of the River Main is a cycle path that follows the river for more than 30km. Its beautiful scenery, good exercise and a real insight into life in Frankfurt.
This was my very favourite weekend activity while living in Frankfurt. Do it, you won't be disappointed!
The parks along the banks of the Main River offer miles of bicycle and jogging trails for the residents of the city. There are also numerous boat ramps and restaurants in the summer months.