Nyköping's main sights are concentrated in the compact town centre, most within easy walking distance of each other. The castle ruins and the Sörmland Museum anchor a pleasant afternoon, while the riverbank and harbour offer easy wandering in summer.
The town is small enough that you can cover the core sights in a day. For those with more time, the archipelago south of town and the Labro Meadows nature reserve on the eastern edge reward a longer stay.