As on your arrival, there are various ways back to the airport
Taxi – Cost will be approximately 45 euros to Charles De Gaulle or 35 euros to Orly. Travel time is about 35 minutes to 1 hour. If you speak some French, you can call Taxis Bleue's special airport line at 01 49 36 66 66. If you call, make sure to mention which airport you will be departing from. The other alternative is to walk down to either Hotel de Ville or Bastille where you will find a taxi stand
Airport Shuttle – Cost is about 22 euros per single person, 14.50 euros per person (2 or more) with travel time varying between 45 minutes to 1 hour 30 minutes. You will need to make a reservation in advance of your trip by calling 01 45 38 55 72 or reserving on line at http://www.airportshuttle.fr. Provide them your flight times and they will provide you a pick-up time. There can be between 1 to 4 stops on their trip. For more details, visit their web site.
Mass transport – By now, you’re an expert. Cost is around 10 euros per person with a total travel time of about 1 hour. If you can stumble through some French, you can find detailed maps at http://www.ratp.fr. Make sure to keep your ticket even after you enter the metro system, you’ll need it to pass through the metro system.
To CDG – Walk to the metro station at St.Paul-Marais (line 1). Purchase a ticket for Roissy – Charles de Gaulle. Take line 1 towards La Defense and exit at Chatelet-Les Halles (2 stops). Follow the signs for the RER B to Roissy – Charles de Gaulle airport. When you are on the platform, make sure the signs indicate the train will be stopping at CDG1 or CDG2. If you are departing from Terminal 1, you’ll need to take a shuttle bus from the RER B station to the terminal. Look for signs labeled “Navettes” and follow the directions to Terminal 1. If you are in Terminal 2, the RER B station is in the same complex. Upon exiting the RER, follow the signs to the terminal and look for the computer monitors to determine which “Hall” your flight is departing from. The total trip should take about 1 hour.
To Orly – Walk to the metro station at St.Paul-Marais (line 1). Purchase a ticket for Orly via ORLYVAL. Take line 1 towards La Defense and exit at Chatelet-Les Halles (2 stops). Follow the signs for the RER B to Orly airport. You may also see indications for ORLYVAL. Take the RER B train to Antony. Make sure the overhead signs on the platform indicate the train will stop at Antony. Upon exiting the train, follow the signs for ORLYVAL and you will take a small rapid shuttle directly to the airport. Your ticket should indicate whether you are at Orly Ouest (West) or Orly Sud (South). The total trip should take about 1 hour.
For other train stations, etc. refer to any of the Paris mass transit maps or the Plan de Paris in the apartment.