Agios Sostis is a small beachfront village on the south coast of Zakynthos, separated from its inland twin Lithakia by a kilometre of olive groves and a single road. Together they form one of the quieter parts of the Laganas Bay coastline — much smaller and less developed than Laganas proper a couple of kilometres east, with most of the action concentrated on the short stretch of seafront and the wooden bridge across to the bar-island of Cameo.
The setting is the eastern edge of the National Marine Park of Zakynthos, the protected nesting area for loggerhead sea turtles in Laganas Bay. From the harbour at Agios Sostis, small boats run trips out to Marathonisi — Turtle Island — a tied uninhabited islet with the bay's main nesting beach. Swimming there is restricted to the cooler hours and away from the nesting strip; on most trips you will see at least one turtle in the water during the crossing.
Lithakia itself is a working inland village, with a few traditional tavernas, an old church and the ruins of a small Venetian-era olive press. Most visitors stay in Agios Sostis on the beach side; Lithakia is where you go for dinner away from the resort menus.
This is not somewhere you fly to Zakynthos for, but it is a calmer base than the louder resorts on the same island. Two or three days fits.