We are flying in to CDG at 8am on a Wednesday and can not go to Airbnb in Bern until Thursday. We would like to stay somewhere a couple of hours outside of Paris that is easily accessible by train from the airport and then to Bern the next day. Since we will arrive so early and probably can't check in anywhere until late afternoon, we'd like to take some travel time off of the next day by heading toward Bern but maybe not going the entire way.
We will be very tired so ease of travelling is very important. We'll also have two teenagers in tow that may need to let off some steam.
Dijon? There looks to be a train from CDG to Dijon, which is a lovely small city with things to amuse you for your short stay. Then a train to Bern (change in Basel) which would take about 3 hours and get you into Bern around 1:00, giving you time to make it to your AirBnB in good time.
Or Strasbourg. Those train trips seem shorter both legs.