I love Patisserie St-Martin! They have a large assortment of things you can buy such as freshly baked bread, deli meats, cheeses, pasta, etc. They have a lovely section of pastries for all kinds of events and I'm sure you can order one for your event.
You can get a sandwich made and even eat it outside. Fantastic.
My only qualm is that the staff is unable to smile, throughout the years I have come here, they all collectively looked pissed off. Sometimes they also tend to keep pastries on show that aren't fresh and on 1-2 occasions wasn't very good....