16. The 63 bus
If you're a tourist in Paris, buying a two-day ticket on a hop on, hop off bus for 24 euros will orient you to the city's geography plus give you a front row seat on all its great sights. For much much less, you can ride the 63 bus from Gare de Lyon to Porte de la Muette and see a lot of the same stuff. Of course, there's no commentary in 27 different languages but the view: as the natives say, "oh la la!"
On its route west from Gare de Lyon, the 63 passes by Jardins des Plantes and the stunning modern Institut du Monde Arabe, winds its way through the Latin Quarter, into the heart of Saint Germain with the imposing Saint Sulpice, by Le Bon Marche in the 7th arrondissement, and then along the quai of the Seine from the Assemble Nationale (with views across the river to Place de la Concorde and the Tuileries) to the Pont de l'Alma. Traversing the Seine, the bus goes by the memorial to journalists above the spot where Princess Diana was killed and up Avenue President Wilson to Trocadero with its splendid views of the Eiffel Tower. Then it's down Avenue Georges Mandel and Henri Martin through the stately elegance of the 16th arrondissement, ending up at the entrance to the Bois de Boulogne.
For art lovers, the route takes you past multiple museums -- the Cluny, site of former Roman baths and home to an impressive collection of medieval art and artifacts, the city of Paris's Musee d'Art Moderne, Asian treasures at the Musee Guimet, and multiple venues at Palais de Chaillot including the recently renovated Cité de l'architecture et du patrimoine and Musee de la Marine.
Buses can be intimidating for first-timers since the stops and routes aren't as obvious as metro stations. But they take the same tickets as the subway and transfers within an hour and a half are free. So buy a carnet (10 pack) of tickets and hop on and off with abandon. Or ride from one end of the line to the other. For 1.60 euro for a single ticket, it's a steal.