4 days in France

I like Avignon a lot. It's 3 hours by train from Paris. Arles is nearby and other towns can be reached by bus.

I might choose Normandy. A few years ago we had 5 nights where we didn't yet have an apartment in Paris and so did that side trip. We trained to Caen, picked up a car, drove to Mont St Michel for a night, to Bayeux for two nights, to Honfleur for 2 nights and dropped the car back in Caen and trained back to Paris. The train tickets were 15 Euro per leg bought 3 mos in advance and printed at home. So it was 60 Euro for the two of for train plus the car. You can see some of our trip here: https://janettravels.wordpress.com/category/normandy/
We spent an evening on Mont St. Michel and a morning to visit the Abbey. On the full day in Bayeux we took an overlord tour of the beaches (we could have driven but I hadn't done the homework necessary to put together the trip.) On the full day in Honfleur we went to Etretat which was amazing. I had seen it in Impressionist art and so put it on the list and am glad I did; it was wonderful.

You could train to Tours and do van tours of Chateaux in the Loire but I would prefer Normandy in the weather this time of year.

Another possibility is Alsace -- you could train to Strasbourg. I am not sure how good transport is to the smaller towns of the region. Strasbourg itself is lovely and worth a couple of days.

As Janet stated Strasbourg or even Colmar are nice places to visit. If you are flying in on Air France, they offer a deal of arranging your train transportation to Strasbourg from CDG as part of your flight purchase.

We rented a car for all these trips outside Paris, and I can't imagine doing it otherwise, though many others have. We took the train from Paris, stayed in one town and drove daily. We never drove back to Paris, but returned by train either to continue our visit or to fly home. You might consider the Loire Valley, even closer to Paris. I thought it was absolutely sensational. We have spent 4-5 in Burgundy, outside Beaune and loved it. This past September we visited Colmar and Alsace. Colmar is beautiful and there are many rewarding day drives ( as was the case in Burgundy as well). We have also spent time based in Arles and combined train and car for about 5 days. I didn't plan on becoming a Francophile, but I think it's happened. We also recently spent time in Normandy (Rouen, Honfluer, Bayeux) and a bit of Brittany, but I think that region might deserve more time, and it was one area where I absolutely would require a car. We visited the Loire, Burgundy, and Arles in late April/May and Alsace/Normandy in late August/September and had great weather and experiences everywhere. Safe travels.

If you choose a place that you can fly to using a "multi-city" plane ticket (such as into Strasbourg or Lyon), that flight connection can be simpler than switching to a train ride upon arrival.

• The faster train tickets are expensive to purchase on short notice and not refundable or changeable if you buy them ahead.
• The regional trains from Normandy are cheap enough to buy on the spot but require other public transport connections in Paris; or you could drive from CDG but there's the issue of picking up a car while jet-lagged. This route is a frequently discussed topic in this forum.

How about Avignon-or another Provence town-for three nights, then train to Lyon for two nights?

Normandy would be easy to do for 4 nights sans car. Train to Bayeux, see the Tapestry and Cathedral etc; the next full day take the Hotel Churchill van trip to Mont St. Michel (book early as it fills up, you don't have to stay at the hotel, although I do like the quaint hotel filled with WWII memorabilia); the next full day do an Overlord tour of the American beaches; return to Paris the day after.

I personally love Burgundy although haven't done it this time of year, but I would not do that area without a car.

As others have suggested have your arrival in France at one of the secondary airports (via a connection in Paris). Some suggestions ...
Lyon >> Annecy (Alps)
Strasbourg >> Colmar (Alsace)
Marseille >> Arles (Provence)
Nice >> Avignon (Provence)
Bordeaux >> Loire Valley (or Biarritz)

The other idea would be to Normandy: Rouen (1) >> Bayeux (2) >> Mont St Michel (1)

In addition to connecting flights to start your trip outside Paris, if you're flying Air France, you can get a connecting train right at the airport (your train gets a "flight number" and they put you on a later train if your incoming flight is delayed). If you search for flights on Air France's website, it will show both options when available. Here's their website about the program: http://www.airfrance.us/US/en/common/resainfovol/avion_train/reservation_avion_train_tgvair_airfrance.htm

The TGV to Avignon/Arles runs straight from CDG. This makes for a lot of traveling in a day, but those are jetlagged hours on a quiet, relaxing train. It would probably take an hour just to get from CDG into Paris.

While I found Avignon a bit moribund, it can be a useful home base without a car. I thought Arles had more sights and character, and was not that much harder to get to. If you can be in Arles for market day, then that's an extra day of activity on its own. You might also luck out and have some nice weather.

Lyon also sits on this rail line and is pretty underrated. An overnight stop here with dinner is a nice warmup for Paris.