For car enthusiasts, there would be the Opel factory in Rüsselsheim, which is just 15km from the airport. However factory tours are only open Monday to Friday.

If you love nature, grab a cab and let them drop you off at the "Schwanheimer Duene", which is 5km to the north of the airport. See this wikipedia article (in German) to learn more about this. Alternatively, take bus 58 in the direction of "Eschborn West" to the stop "Höchster Markt" and then bus 51 in the direction of " Trift-/Frauenhofstraße" until the stop "Alter Friedhof Schwanheim". From there it is just a short 10 minute walk. On your way back, stop in Frankfurt-Hoechst and grab something to eat in the old town around the "Hoechster Schloss".