Visit the old "Speicherstadt" (storage city) from seaside, doing a boat tour on one of these smaller ships called "Barkasse", starting from St. Pauli Landungsbrücken.
Once the biggest warehouse complex of the world. There are museums :
Hamburg Dungeon -
Dialog in the darkness
the old harbor district of Hamburg, with a lot of storage halls. Earlier it was a duty free zone, you have to enter or leave go through customs control. In front of the halls boat harbor tours starts
For the money (about $10 or less) the harbor cruise is well worth it. The old warehouse district is fascinating all by itself.