The Post-Byzantine Aqueduct is a small but satisfying walk from Ouranoupoli. From the centre, follow the coast for about half an hour, then turn inland where the path leads toward the old waterworks.
The ruins are not a major archaeological site, but they make a good low-key excursion if you have time before or after visiting the harbour. The walk is part of the appeal, with sea views on the way out and a quieter landscape beyond town.