Nice day at the beach
You can rent a basket for the day for ~10 Euro that includes a lock so you can swim or walk around with your stuff safely stowed.
Rostock has a long-standing association with tall ships, and aficionados could time a cruise to coincide with the annual Hanse Sail Rostock in August, when 200 or so square-riggers and other classic sailing boats sail into port for four days of regattas, fireworks and markets. This event draws local sailors, who moor for the festivities, along with hundreds of thousands of visitors.
Most likely a great summer resort but works just fine for some relaxation in December too. A stroll along the "canal" and the fishmarket is also worth a visit.
From 6th until 14th July 2019 the 82nd edition of the maritime festival and sailing event will take place. Warnemünde Week is among the largest international sailing events in Germany. About 1,800 sailors from around 30 nations meet annually in Warnemünde for high-powered sailing as well as surfing and kiting competitions on the water. In addition, the cultural supporting programme also attracts more than 650,000 visitors to Rostock’s seaside resort every year. A major festival parade with traditional costumes for the opening ceremony, festive supporting events, sports offers such as a beach handball tournament, show programmes, live concerts, dragon boat festival as well as a “shanty & folkloric costume gathering” will make guests and sports enthusiasts equally active.
Ein Urlaub in Warnemünde lohnt sich zu jeder Jahreszeit. Im Sommer ist an der Ostsee Hochsaison. Große Veranstaltungen wie die WARNEMÜNDER WOCHE oder die HANSE SAIL locken Gäste in unser Ostseebad und nach Rostock. Auch für Kreuzfahrtschiffe ist Warnemünde ein beliebter Hafen.
Wenn es in Warnemünde ruhiger wird und der Trubel sich gelegt hat, kann man die Ostsee und den Strand auf ganz besondere Weise genießen. Im Herbst und Winter ist es hier ganz wunderbar zum Abschalten und Entspannen.
There's an old Stasi prison you can visit in Rostock. It can be a bit difficult if you don't understand German, as from what I recall, I think most, if not all of the exhibits, don't have any English translations (correct me if I'm wrong).
You can also climb the lighthouse in Warnemünde and get a pretty great view.