If you have at least 3-4 hours free time , you can take Taxi to paris .Don't waste time with RER (train) or bus etc . Make sure leave your heavy luggages with airport concierge and calculate the « real » free time you have before being back in airport .
You can visit famous monuments and walk in touristic center of Paris or even take double -decker bus tour etc , depending on how much time you have .
Taxi ride from center of Paris to CDG at late night or early morning will be about 35 minutes and in normal day time about 40-50 minutes ( still check the traffic condition and rush hour commute time ).