Archenhold Observatory sits in Treptower Park and is best known for its enormous 1896 refracting telescope, a 21-metre instrument with a 68 cm lens. It is now part of Berlin's technical museum world, but still feels like a specialist stop for anyone interested in astronomy, engineering, or scientific history.
The observatory also has a strong Einstein connection: he gave one of his first public lectures on relativity here in 1915. Combine it with a walk through Treptower Park rather than treating it as a quick central Berlin sight.