Any chance you could stay one or two days post cruise? HAL has hotel program in Montreal if you don't want to book on your own..... or you might be able to get a better price booking yourself. The ship docks right in Old Montreal so not a long taxi ride to any number of hotels. You would have to arrange your own transportation to the airport but that is easily accomplished.
IMO, Montreal is really worth it. In the past, we have taken long weekend trips there and even in freezing cold February, enjoyed it.