I recommend to go to Timmendorfer Strand (1 houre by car). It has a nice beach and a Sea Live Aquarium. Even if the weather is to cold to go swimming, the beach is always nice for a walk.

http://www.timmendorfer-strand.de/en...tion/home.html

http://www.visitsealife.com/Timmendorfer-strand/en/