ijgordon

Last year I had a 12 hour connection and went into the city for the day, and there was plenty of time for a handful of activities, including shower/breakfast at the AF arrivals lounge, lunch, and a trip up the Eiffel Tower, as well as stopping at Gare du Nord in both directions to leave my carry-ons in a locker (the luggage storage at the airport closed too early in the evning).
For 10 hours, I would absolutely still go into town.
I think it's only ~40 minutes on the RER to Les Halles, and they do run express trains outside of the rush hour (they go nonstop after Gare du Nord) that save ~10 minutes (if you can avoid rush hour entirely on the RER, that would be preferred!)