The aqueduct that supplies Óbidos with water runs south from the town across the surrounding fields on simple stone arches. It's a sturdy, modest piece of 16th-century engineering rather than a Roman showpiece, but a good loop walk takes you out the Porta da Vila and along the base of the aqueduct for about half a kilometre.
Combine the walk with a visit to the nearby Capela de São Martinho, a tiny chapel that sits at the foot of the walls and shows up in early photographs of the town.