Anna, I didn't cruise it, but I've been to Halifax, Charlotte, Quebec & PEI
I'll just jump right in.
Halifax is lovely, but it's HILLY! I mean "streets of san francisco" hilly. I mean walking at a 45-degree angle, "killing your Achilles tendon if you continuously walk uphill"-hilly.
Lots of things are at the TOP of the hill (fort, for example, with events at certain hours), while interesting shops & restaurants are at the BOTTOM of the hill at the harbor. So read about the town, make a decision where you want to go and route appropriately, so you're not walking up and down and up and down and up and down.
Quebec is beautiful....but again, it's built on two layers. A funicular can take you up and down the hill and most of the historical aspects are within the walled city if you want to limit the time you spend. We walked up the hill from the train station (took that from Montreal) and while it was steep, it didn't take all that long to get to the walled portion (~20 minutes). We didn't find out about the funicular until we were at the top. Then we rode it down and took a harbor cruise...not sure how much time you'll have in the city.
Charlotte...very pretty---everyone there must have the gardening gene. It's flatter and easily walked. We did a walking historical home/garden tour with tea. PEI is gently rolling hills and ocean views, but we saw all this from our rental car. We went to the theater during the evening, unfortunately, I'm not sure how much of the town you can see on public transport.