In one of my passages through Frankfurt, I went to Bamberg. If you're into beer tourism, this is a great visit and quite affordable. I'm not a big beer drinker (2 is my limit), but this place is an exceptional experience. I took a slow regional train, but there are faster options. Total transit time 2-3.5 hours depending on train schedules, less than Munich or Salzburg (which is just incredible BTW and should eventually be visited). Beyond that, most of my experience is around Bundesliga stadiums. I tend to need only fill 4-5 hours around any particular city. Even Frankfurt has enough to fill a half day itinerary.