We rented cars in Saint John, New Brunswick and Halifax, Nova Scotia for our party of 4 during our Canada/ New England cruise about 7 years ago. There are car rental locations close to the cruise terminals. Not sure how expensive or how available rental cars are currently. Depending on how long you are in port, it may be worth checking into for DIY flexible touring.
In Saint John we drove out to St. Martins, drove part of the Fundy Trail Parkway, which was very scenic, stopped at the Sea Caves, and ate at a seafood restaurant near the caves.
https://fundytrailparkway.com/
https://destinationstmartins.com/
In Halifax, we drove out to Peggy's Cove, and made several other stops including the Citadel in Halifax. We could have stopped at the cemetery, but decided not to.