The Santo André and Sancha lagoons add birdlife, reeds, dunes, and Atlantic air to a Santiago do Cacém day. The reserve protects a chain of coastal wetlands where inland Alentejo gives way to beach country, and it is especially worthwhile for walkers and birdwatchers.
It sits outside the town rather than in it, so it belongs on an itinerary only if you have a car. Pair it with Miróbriga and the castle for a day that feels varied rather than dutiful.