Ancient Trizina, near modern Troizina, is an archaeological stop tied to the myths of Theseus and the older history of the Saronic Gulf. The remains are scattered rather than monumental, but the setting gives the site a quiet pull.
It works well as part of a day away from Poros town, especially if you combine it with Galatas and the nearby countryside. Do not expect a polished major ruin; come for the layers of local myth and history.