We're going from Switzerland to Paris but we have an extra day or two to spend somewhere in between. Any suggestions?
Where in Switzerland will you be? I'd check the train line from that location to see options.
Without knowing where you are coming from, I’d personally pick Lyon or Colmar/Strasbourg as you can then easily continue journey on to Paris
Thinking about Dijon.
What time of year? Annecy is lovely; Lyon is larger with more to see and do and would be a great two night city -- in fact we spent two nights there as a side trip from Paris. Get a guide for the traboules if you go because many locals closed off access during COVID and some guides have better access than you will have just on your own -- although there are still many open to tour individually. You can get a map at the Tourist office.
Stay an extra day in Switzerland and catch a budget airline (cheap) into Paris.