a layover of 3.5 hours in Frankfurt

No, not really. Once you land, it will take you about 30-45 min. to emerge into the airport, depending on where you are arriving from. An inter-Europe flight doesn't take as long usually. For your next flight, again, it depends on where you are flying to. If it is in Europe, you should be at the airport going through security about 1.5 hours before departure. If it is the US, then 2 hours. About the only thing you might have time to do, is walk over to the Squaire which is above the long distance train station and admire the unique architecture. Grab some nice pastries from one of the good bakeries like the Marche' Cafe' or Kemps, or a wrap from Dean and Davids under terminal 1, at the food court.