There are two métro stops in the surrounding area, according to about.com - Les Halles/Châtelet (line 4), which I've marked with a red star, and Hotel de Ville (line 11), which has a green star.
On that day, if the metro hadn't been so crowded, we would have taken it back to our hotel - which would have meant that we needed to get to the the station at Odéon, which I've marked with a blue star.
The most straightforward route would have been to get on an outbound train at Les Halles, because that stop is on the line that also goes to Odéon - the Porte d'Orléans line, or line 4. We'd have gotten on that one train, and taken it five stops.
We could also have started on the Hotel de Ville stop. We'd have gotten on that brown-line train and taken it only one stop, to where it intersects with the purple line, line 4, on the map. Then we'd have transferred to line 4 and taken it four stops to get to Porte d'Orléans.
Also, here is what a ticket looks like! Photo credit goes to http://www.parisdigest.com/Photos_new/metro_paris_ticket.jpg.