The Archaeology Museum of Baden-Wuerttemberg gives Konstanz a deep-time counterweight to its lakefront and medieval lanes. Its exhibits cover prehistoric settlement, Roman traces, early medieval finds, and the long human story of the Lake Constance region.
It is especially useful if you are using Konstanz as a base around the Bodensee. The museum helps explain why this corner of Europe has always been a crossing point, with water routes, trade, and borderland cultures layered on top of one another.