The Sangiran Early Man Site Museum anchors one of Southeast Asia's most important fossil landscapes. Finds from this area have shaped the study of early humans in Java, and the museum explains the archaeology, geology, and Homo erectus discoveries in a way that makes the wider landscape intelligible.
Sangiran sits outside central Solo, so it is best treated as a half-day excursion rather than a quick city sight. The payoff is substantial if you have any interest in prehistory: few places near Solo connect so directly with deep human time.