If you stay several days in Paris, unless you decide to use the cheap and convenient Vélib (the Parisian bike sharing network), you will probably use the wide offer of public transport.
Buses can be a good option (especially the line n°69, connecting several important monuments or tourist sites), but has several disadvantages, notably long journeys and low frequency. The metro, on the contrary, is quick, frequent, and easy to use (except with a wheelchair or a pushchair). But it is mostly underground, and doesn’t offer any interesting landscape… Except in a few places !
Indeed, lines 2 and 5 are partly elevated, and line 6 even runs outside on 45% of its length, above the Boulevards Extérieurs.
If you use line 6, try to take the time to overlook the city, from what would be the second or third floor of the buildings. Starting at Nation in the East, the line appears a couple of times outside before crossing the Seine on the beautiful Pont de Bercy, offering a view on the two contemporary districts Bercy and BNF. Further, there’s another open air section between Place d’Italie and Denfert-Rochereau.
But the best part is definitely the Western side, between Pasteur and Passy, where you can enjoy a view on the Invalides and the Eiffel Tower, see the beautiful metallic architecture of the elevated stations and enjoy a gorgeous view over the river when you cross the Bir-Hakeim bridge. Not by chance the favourite place for Marlon Brando in Last Tango in Paris…