Since you are on a cruise how much time do you have at each port?
I can only comment about Montreal and Quebec City.
Montreal: Old Montreal, Mont Royal park; Botanical Gardens though I haven't been there in decades to really recall what it was like.
Quebec City: just walk around the upper town and lower town- so much to see! Lots of museums-I really like the Musuem of Civilization, the Citadel- home of the "Van Doos" 22nd regiment - I ditched my tour because of a late spring snowstorm-it's mainly outdoors and requires a ticket- they can be purchased starting 30 days prior.
Montmorency Falls, it's a short trip to the outskirts of the city - only 13 km, a trip to Ile d'Orlean if you have more time.
It's been years since I was in Halifax and my time there was brief- it looks like a lovely small city. Peggy's Cove is about 1 hour or so, by car, south of Halifax-.