Paris and beyond in 8 days?

I would definitely take a little venture out of Paris. You could spend weeks there and not see it all, but you can spend a few days and see the highlights and be happy with that view of Paris. I would rather leave someplace wanting more than thinking I should have seen something else. I would take the train to Strousburg or Avignon. Both are great places that would add to your experience of France. If you go to Avignon you might get a car for a few days and explore Provence. Have a wonderful time!

I think Avignonor better yet, a smaller city like Arles or Nimesis a splendid suggestion, and there are direct trains back from Avignon that go straight to Charles deGaulle without stopping in Paris. Another possibility might be Beaune.

You have received good advice so far, but don't underestimate how much there is to do in Paris. I have stayed a total of 15 nights in two trips and still have not done quite a few things I want to do. Also there are easy daytrips from Paris, such as to Versailles and Giverny. What are you and your friends interested in? That might narrow it down for you. Besides the places mentioned in France, it is easy to get to London from Paris on the Eurostar. No car required to see London!

Loire chateaux would be a great trip for 3 or so days. Plus Chartres

Thank you everyone for all the suugestions. As to what we like to do. One of us has been to Paris many times, one was recently there for three nights and the other has never been. We are looking forward to the food, wine, French culture, museums and the history. Within Paris itself we will have no problems filling our days by walking the streets and taking it all in as it comes but thought it would be fun to have a taste of France beyond Paris. Thank you for the suggestions so far - there are some good ones. We look forward to hearing others.

If you are interested in 'recent' history, what about a tour of the Normandy landing beaches? Another interesting place just south of Calais is the Block Haus d'Eperlecques. It is where the Nazis had the V1 and V2 rockets assembled for 'taking out' London during the latter days of the 2nd world war. The bunker is complete. You do not have to do a tour, you can just wander around in your own time. The whole thing is very well done and is a very moving experience.
If this all sounds a little depressing for a 'holiday', then all of the above suggestions are good, especially Beaune.

Lots of good suggestions here; since you're going in September and it's probably time for the wine harvest, perhaps Colmar via Strasbourg might be fun - or another wine growing region.