Quebec City has flagship Canadian jeweler Birks (the Canadian equivalent of Tiffany & Co.). There is also a Holt Renfrew which has a high end jewelery section. The old town of Quebec City has many small, independent jewelers with lots of dazzling diamond jewelery. All the same can be said about Montreal.

Should you choose to stay in Quebec City, the Fairmont Chateau Frontenac is a luxurious choice. We were lucky to get a suite with it's own castle turret: http://canuckflyer.itgo.com/chateaufrontenac.html
It truly added to the romantic experience!

Enjoy your trip, wherever you go.