Hi cmjb4ever, and welcome to cruise-chat.com. I've done this itinerary on Carnival twice - on C. Destiny and C. Victory - and on other lines four other times. Halifax is among my favorite ports.
As with all shore excursions, a lot depends on what you enjoy to do. If you are into history, Halifax is a gold mine, between the Maritime Museum of the Atlantic, the Pier 21 Immigration Museum, and the Citadel. Want to learn more about Titanic? (The real thing...not the movie.) Halifax was the site of the recovery, and there are three cemeteries in which many who perished are now buried.If you like pretty scenery, a trip to Peggy's Cove might be nice.
Same thing in Saint John. The reversing falls are a great attraction - you can brave them by boat, if you wish, or just observe from above. There are also excursions that take you out to areas in which you can see how quick the tidal action is.
Let us know more about what interests you and we can help you narrow your choices.
You can also do a search in our Ports of Call section for more info on these ports.