The Gründerzeitmuseum is a period-room museum devoted to Berlin's late-19th-century Wilhelminian style. Instead of a conventional display case museum, it preserves furniture, interiors, and domestic objects in rooms that show how prosperous Berlin households wanted to present themselves.
Visits are by guided tour, which makes it less casual than the big central museums, but worthwhile if you like interiors, design history, or the quieter edges of Berlin's museum scene.