The Cap San Diego, moored at the Überseebrücke a short walk east of the Landungsbrücken, is the largest seaworthy cargo museum ship in the world. Built in 1961 for the Hamburg-South America Line, she made 120 round trips to Buenos Aires before retiring in 1986 and being preserved by the city.
You can visit the entire ship — bridge, engine room, crew quarters, holds, cargo hatches — and the layout makes the daily life of a merchant marine vessel surprisingly vivid. She still sails several times a year for short cruises in the German Bight; check the schedule if you want to ride a working break-bulk freighter. There is a hostel on board for adventurous overnight stays.