If you're arriving via Eurostar to Paris Gare du Nord, then bus 42 is perfect for sightseeing. The route begins right next to the station (so no chance of taking it in the wrong direction) and goes past Opera, Madeleine, Place de la Concorde, part of the Champs-Elysees, and to the gardens next to the Eiffel Tower. Public bus, so it is inexpensive as well!