Terrasse Dufferin is an expansive wood plank walkway that wraps around the Château Frontenac towards the Citadelle, overlooking the St. Lawrence River. It's full of gazebos and benches and interesting people. I took my walk here each morning.
Beneath sections of the terrace are the remains of Saint Louis Forts as well as Château Saint-Louis, residence for French and British Governors. Admission is free.