we are planning a 10 day stay, arriving in Paris in late Sept/early Oct. traveling as a family with 2 adults with a 10 year old and a 12 year old. would like to take the train out of Paris and finish the second half of the trip with a smaller French countryside visit for 3-4 days. looking for a beautiful spot for a little relaxing before returning home. considering Nice, Annecy or Aix-en-Provence, but very open to suggestions.
You could get an apartment for 10 nights in Paris and then do 2 or 3 days trips to small towns nearby without having to move. e.g. Senlis, Auvers sur Oise, Moret su Loing and many more.
if you do decide to go to a second city then start there. you don't want to waste your second to last day which is prime vacation tme on the logistics of getting b ack to Paris. So maybe go directly by train to the smaller site on arrival and use that miserable jet lag day to get to your furthest destination.
Nice is a big city. And a long way from Paris. Lyon is also a city but its central area is manageable like a small town. Or Annecy is gorgeous. Strasbourg would be nice base. lSt. Malo is delightful. then finish in Paris.
Can you fly home out of Geneva (Annecy) or Nice/Marseilles?
Do you like wine? Alsace (half timbered villages), Burgundy or Loire (and castles) are quite nice.
Lengthen your summer by departing (flying out) from Nice and spend the out-of-Paris part of the trip in Antibes (next to the airport) with the whole coast from Cannes to Monaco to choose from for those days (via local train or bus or driver).
The Alsace area sounds perfect. You can go directly from CDG by train in about 2 hours if you want to start your trip there. Or you could fly into Basel (1.5 hours away) and fly home from Paris.