Most rental cars will let you drive one-way with no additional fees. Europcar and Sixt had no problem with me doing it that way.. and both have offices in Coberg (I just checked)... I had two separate weekend special rentals and I had no problem switching cars between Frankfurt and Munich. It's a much easier way of doing it.. and it's about $90 a day. Trains are expensive in Germany, but FRA does have a train station right underneath it. Not sure about Coburg.