The Hanseatic city of Rostock on the Baltic coast and the UNESCO city of Stralsund offer fascinating insights into the history of maritime trade, as well as some stunning architecture. The spectacular chalk cliffs on the island of Rügen are a favourite among visitors too.
Rostock is often described as the "gateway to the north." More: http://www.dw.de/discover-germany-the-travel-guide-2014-02-01/e-17346491-9798