Unfortunately we just flew in and out of Frankfurt and never actually spent any real time there. The hotel across the bridge connected to it is pretty nice and fancy but that is all I know.
Heidelberg is about 30km away from the airport so you'd have to take a train or bus, but it's a nice historic town. Also a major university so lots of bars/restaurants etc.Heidelberg - Wikipedia, the free encyclopedia
Speyer is about the same distance and has a very well known technology museum.TECHNIC MUSEUM SPEYER