Frankfurt as a Stop-Over
You found a cheap flight with a stop over in Frankfurt am Main, Germany? Great, use the undesirable pause to visit a few places of interest.
2 Hours Stop-Over
Stay in the airport!
4 Hours Stop-Over
Pray with a minyan!
Frankfurt has a daily Shacharit and Mincha/Maariv minyan, located in the Westend-Synagogue (inside the Shtible). For time please see here.
Visit a Kever Tzaddikim!
In Frankfurt are buried quite a few great Tzaddikim. If you have a chance make sure to visit them.
Very old cemetery Batton Street: Rabbi Natan Adler, Rabbi Pinchas Halevi Horowitz, Rabbi Jakob Joshua Falk - (Penei Yehoshua), Mayer Amschel Rothschild
Old cemetery Rat Beil Street: Stoliner Rebbe, Rabbi Shimshon Rafael Hirsch
8 Hours Stop-Over
You can tour Michelstadt or Worms and Speyer. I dont know if you can make everything in 8 hours, the best is you call one of the taxi drivers for more details.