In addition to Saint John (no "s") and Halifax, both Sydney and Charlottetown get considerable cruise ship traffic as well. Some cruise ships go to Quebec City and even Montreal. The odd cruise ship even makes it to St. John's (note the "s").
Not only do passengers get out and wander around the historic ports, there are often excursions to nearby attractions like the Fundy Trail, Fundy National Park, Hopewell Rocks, Peggy's Cove, Green Gables, Fortress Louisbourg etc (depending on your port of call).
The cruise ship industry is pretty big on the east coast and it can be really neat seeing three (and sometimes even four) cruise ships berthed at a time in Halifax and Saint John. The season extends well into October for the "leaf peepers".