The Sörmland Museum occupies the reconstructed buildings alongside Nyköping Castle, making it the natural companion to a visit to the castle grounds. It is the cultural-historical museum for Södermanland County, covering the region's archaeology, applied arts, and folk traditions from prehistoric times to the present.
The permanent collection is strongest on medieval history — which makes sense given the castle's significance — and includes textile collections and local craft traditions. The museum also puts on temporary exhibitions drawing on national collections. Entry is free, which makes it easy to pop in even for an hour. The café in the castle courtyard is a decent spot for a coffee between sights.