Bremerhaven is Bremen's port on the Weser estuary, built for ships too large or too ocean-bound for the old inland city. It is a working harbour first and a pretty town second, which is exactly why it is worth adding to a northern Germany trip.
The best sights sit close together around the Havenwelten waterfront. The German Emigration Center tells the human story of millions leaving Europe by ship. The German Maritime Museum and the preserved submarine Wilhelm Bauer cover the hardware of seafaring, while Klimahaus Bremerhaven turns climate science into a full-scale visitor experience.
Bremerhaven is not as handsome as Bremen or Hamburg, and it does not pretend otherwise. Come for migration, ships, docks, polar bears at Zoo am Meer, fish, wind, and the blunt mood of a North Sea port.