We did this cruise last July, stopping in Halifax and Saint John. It was a very good cruise...it was also our fifth cruise in the region, so I'm somewhat biased about the itinerary, but I will say that our experience on the ship exceeded expectations.
I have a review around here somewhere... I wrote it last year soon after we returned, but never got around to submitting it. I'll let you know when it's posted. But, until it is, here are reviews from past cruisers who submitted theirs in a more timely fashion:
As far as excursions, tell us a bit more about what you generally like to see -- are you into museums, scenery, history, water activities? Both cities have something for everyone. And BTW, the weather is far from cold in this part of Canada during the summer, so no need to be concerned.