One night stay accommodation in Paris

Pretty much all of central Paris is between Gare Montparnasse and CDG. What do you hope to see/do on that final night? That might influence where you stay. The travel time to CDG will depend on the time of day. I would plan at least an hour.

hey hey lekasha
how many is "we", budget in euros, exact date, what time is flight and is it back to USA/Canada to help you more. time to get to airport could very with traffic, accident, picking you up on time, look up taxi (G7.fr) to reserve or have hotel reserve for you. good luck
aloha

Hotel Pas de Calais in the 6th arrondissement. (Or somewhere in the neighborhood.) Fifteen minute walk to the Tuileries in one direction and 15 min walk to Luxembourg Gardens in another direction. Take a taxi from Montparnasse to the hotel and use the G7 Paris taxi app a couple of days in advance to request a taxi (fixed rate to CDG) at the appropriate time for your departure to CDG. It took us an hour to get between our hotel and the airport on a Saturday afternoon, but it depends on when you are going. I'm sure others can give a good timeline for departing for the airport.

Nothing wrong with the Montparnasse area which is within walking distance to Luxembourg Gardens or a #91 bus ride over to the Botanical Gardens near the Austerlitz train station and the banks of the Seine. You are within walking distance from the train station to the Vavin metro stop, a lively and lovely residential, as well as commercial area. Here's a Mercure at the corner of Boulevards Montparnasse and Raspail.
There are many more in the area, such as Villa Modigliani. If I remember correctly, my husband and I stayed at the Mercure for one night before taking a cab to the airport for a flight to the US. We used to live in the area and have stayed in this area many times. We've never stayed at Villa Modigliani but it has always looked interesting to us.
If you want more, just search using metro Vavin as a search word.

What time does your train arrive from Brittany? I shouldn't think it would be that early in the day. By the time you settle into a hotel, adjust and repack luggage for the trip home, and arrange for CDG transportation early the next morning, I'm not sure how much relaxing is going to be involved.

This is what I would do. It's not really even a recommendation necessarily. For that, you would need to provide a lot more information.

• Book a nearby hotel with spa, either Le Six or Bonsoir Madame (both have small spas and are a short downhill walk form Gare Montparnasse).
• Stop by Marin Montagut for some last minute gifts or souvenirs.
  
• Arrange for a relaxing massage in the spa before dinner at 20h.
• Book several days in advance for the evening meal - probably La Gentiane, Wadja, or maybe even la Coupole.
• Spend the afternoon at the Jardin du Luxembourg. Depending upon the time, maybe continue to the Jardin des Plantes, the Alpine Garden, and perhaps take métro 10 from there to Jardin Albert Kahn.
  
• Call Taxi G7 for an early morning pickup to CDG (fare is fixed at 72€). The pickup time should be 4 hours before your CDG departure, 4+15 if it might rain.

Once I had all this arranged, I would be able to relax and enjoy my final evening.