Hello from Wisconsin,
Renting a car at CDG is easy, getting out of CDG airport with your rental and on to 'your road' is difficult. No sign will be provided for your destination. However, returning a car to CDG is a piece of cake. All signs point you to Car Return. Follow the signs and your car will go to its home.
Plan ahead to avoid 'bad', crowded roads when returning to CDG. You might have to take a bit of a circuitous route to the airport. Absolutely do not try driving in or through Paris. But these outer ring roads are interstate quality and easily navigated. Don't get too close to Paris to start your descent into CDG airport.
Returning the car early and staying in the Marias is a great idea. Take the subway from the airport. Paris RER operates a train from Aéroport Charles de Gaulle 2 to Châtelet les Halles every 5 minutes. Tickets cost €5 - €9 and the journey takes 38 min. The walk from Les Halles to the Marias is an easy walk. Flat, paved, occupied. You will likely pass by the Pompidou Museum. The beauty of taking the RER from the airport is that it is the terminus for that end of the line. You can not possibly hop on the train and have it go the wrong way. Getting off at the correct stop is also fairly easy. The 11th stop after getting on is Gare du Nord expect a lot of people to exit at this stop. The very next stop will be Châtelet les Halles.
Get off and get above ground and orient yourself by either using you phone' mapping, or by having a quality, detailed paper map.
enjoy,
wayne iNWI