A cafe terrace is the place to go, of course, but choose it carefully: not too much sun, not too much shade, not too much traffic noise. Some of the best are on the Left Bank; for a concentrated shot of what has always made Paris Paris, everyone should experience the Deux Magots or its neighbouring Flore (if you can bear spending that much on a coffee) once in their lives. Remember: Verlaine, Mallarme, Beckett, Faulkner, Fitzgerald, Dali, Eluard and Picasso were there before you.
170 Boulevard St Germain; Metro: St-Germain-des-Pres