Many choices. Tour of the Citadelle, walk around Plains of Abraham (including boardwal), either walk or take funicular from Dufferin Terrace to Lower Town, see Chateau Frontenac, Quebec Legislature and area.
https://www.lacitadelle.qc.ca/en/informations/schedule-and-fees.html
If you're an Inspector Gamache fan, try this:
https://www.gamacheseries.com/bury-your-dead-real-place/