Hi Jan:
Sorry, just found your post. Why do you want to rent a car. The tiny seaport of Warmemunde is very quaint. Also the train station is next to the port and you can take it to Berlin. The town of Rostock is old and interesting but if you only have a day and have not seen berlin lately, the train would be easy and then use the underground to get around in Berlin. If you went a car you would have to drive pretty far out of Warmemunde and Rostock to see anything much.. Feel free to email for more discussion
PAUL