Stick to the highway, traffic and the roads are a mess in that area--more so, in my mind, than on the eastern side of Paris.

You'll need every bit of your attention for the road conditions--visit La Défense another time via the metro. It's never quiet that side of Paris, but avoiding rush hour times would definitely help.

Can you return it anywhere else? CDG?

IIRC, you'll see La Défense in the distance before you enter the tunnel. You'll come out of the tunnel in Neuilly-sur-Seine on avenue Charles de Gaulle and just keep going straight through the Porte de Maillot and up avenue de la Grande Armée to the Etoile and then the first exit is avenue Foch.

Can you possibly return the car in Vernon after you visit Giverny and take the train into Paris? We did that - it will work if you don't have a lot of luggage.

I just asked my husband about this. As well as staying 'stick to the highway' (which you've already been told!) he also says to not look too hard on the actual avenue for the Europcar office: it's located in an underground car park at the location

Good advice from Bianca... spot the entrance using Google Streetview before you go. http://g.co/maps/r3w7x

CDG is a looong way out of the way in the wrong direction so
forget that idea.

Thank you all. We will be arriving on a Saturday, midday, so hopefully the traffic won't be too bad. Appreciate the tip that the EuropCar office is not visable from the street!

I'm with Gretchen, I think you should return the car to CDG and take the train or a taxi into Paris. I drove into Paris once and vowed never to do it again.