Rick Steeves' Paris recommends several "Scenic Buses for Tourists." I'd like to add one more: Bus 84. It's roughly equivalent to a Manhattan "crosstown" bus. It stops at the Parc Monceau, Church of the Madeleine, Place de la Concorde, Musee D'Orsay (bus stop is about a block or two away), St. Sulpice, the Luxembourg Gardens and the Pantheon. It doesn't run on Sundays and holidays.