The Nyköpingsån river runs through the centre of town before opening into a small harbour at the Baltic coast, and the walking paths along both banks are the most pleasant way to move between Nyköping's sights. The waterfront around the castle is particularly good in summer — the old wooden warehouses that line the south bank have been converted into restaurants and bars, and the harbour mouth draws yachts from across the archipelago.
Follow the river downstream from the castle towards the sea and you pass through a quieter stretch of parkland before reaching the harbour, where a footbridge crosses to the south side. The return walk on the opposite bank brings you back through the old town. The whole circuit takes under an hour at a comfortable pace and gives you a feel for the town that the castle alone does not.