One day
Start on the Alter Markt with the ornate Old Town Hall and St Nikolai church, then wander the brick-Gothic old town to the Neuer Markt and St Marien, climbing its tower for the view. Have a harbour Fischbrötchen for lunch, spend the afternoon at the Ozeaneum aquarium and the tall ships on the waterfront, and end with a fish dinner and a harbour-side stroll.