When I travelled (July 2009), a one-day pass for all bus and metro trains was CAD$9 with a pass for three consecutive days being CAD$17, which is actually extremely good value. To buy your ticket, ask the attendant for a ‘Carte Touristique’. The ticket allows you to go anywhere freely across the city. To use it on the metro, take it to the ticket office and show the attendant and they will let you through.

When you arrive in Montreal (and buy a Cart Touristique), its a really good idea to ask for ‘un Plan de Metro’. They are free and a small, pocket-sized guide to the Metro which will be really useful to you.