Montparnasse

Montparnasse is the same deal as Montmartre: it is a destination in itself. If you have to choose between the two for lack of time, I would go the Montmartre route, but it does require a larger portion of the day than Montparnasse. I got a kick out of Montparnasse because of having read The Sun Also Rises, which features the cafés that have made the area famous, like the Select, the Rotonde, and the Dôme. Hemingway, Joyce, and many of the Montmartre painters spent a great deal of time in Montparnasse, but it is no longer the case with current writers. There is, however, a very beautiful park nearby that is worth seeing. I cannot recall the name, but it is not difficult to find.