In the photo you can see the original entrance to the 'metropolitain', a design of Hector Guimar. Thanks to its exceptional public transportation network, moving around Paris is externally easy. The faster, and more convenient, transport is the 'metro'. Although most of the lines run underground, there are 2 lines with which you can still enjoy the landscape, as they are running outside ('metro aerien'). They are going round Paris by North ('via Barbes') and South (Via Denfert). You can take them both from "Nation-Charles de Gaulle-Etoile".
If you plan to stay in Paris several days, you should buy a 'Carte Orange', which let's you take all public transportation during a week - from Monday through Sunday. If you are only going to stay one day, buy a one day ticket, you will save a lot of money with it.