Tea and pastries!
The card you see in the picture above in the bottom left is my Carte Navigo. If you're in Paris for longer than a week and plan to use the metro a lot: get one. If I understood the lady at the desk correctly, though, you have to pretend not to be a tourist. :). Also, if possible, make sure to be in Paris during the "Fete de la musique". A great evening during which there is music at every street corner, and people are out and about: dancing, drinking, etc. I spent it at Montmatre and jumped at techno music right next to the Sacre Coeur cathedral. Fantastic!