If you have a long layover at Charles de Gaulle airport and you are sitting in the terminal wondering whether to go into the city, the answer is yes. Here is exactly how to do it in three hours using only public transport.

Leave your bags at the airport. The consigne at CDG costs around 5 euros per bag and it is completely worth it. You cannot navigate Paris efficiently with a suitcase.

Buy a day pass for the Ile-de-France transport network through the app before you leave the terminal. The app is called Ile-de-France Mobilites. The pass costs around 29 euros and covers the RER B from the airport and all buses and metro once you are in the city. Do not queue at the machines in the terminal. That alone saves you fifteen minutes.

Take the RER B from CDG Terminal 1 or 2. Get off at Chatelet-Les Halles in central Paris. This takes about 30 minutes. Use Citymapper, not Google Maps, for navigation in Paris. Citymapper handles the bus and metro connections more accurately.

From Chatelet, take bus line 72 along the Seine. This is the secret. Bus 72 runs on the surface through the heart of Paris and passes the Louvre, the Tuileries, Place de la Concorde, the Grand Palais, and the Pont de l’Alma. You see all of this through the window for the price of a regular bus ticket.

Get off at Pont de l’Alma and walk to the Eiffel Tower. The esplanade is now pedestrianized and you can walk right up to the base. Take your photos and get back on a bus.

From the Eiffel Tower, take bus 30 toward Pigalle and walk up to Montmartre. The climb is steep but the view from the Sacre-Coeur looking out over all of Paris makes every step worth it.

Total time from airport arrival to return: approximately three hours. Total cost: the transport pass. Everything else is free.