For first-time visitors without much time, a bus trip is ideal to orient yourself if you don’t mind the groupstyle travel.
Paris Opentour is in comfortable double-decker buses, with a live or recorded commentary (in whatever language you want).

Use the Opentour Paris bus pass to discover the city for one or two days. You can buy your Opentour Paris Hop On Hop Off pass in advance and have it delivered to your home, your hotel, or a convenient pickup location in Paris if you’re renting an apartment.

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Other online ressources: Opentour