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 in 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 , number of places you can go depends how much time you have .
At night or very late night you will Find famous Cabarets , nightclubs and international bars , cafes and restaurants , you can find them all in Google . Specially in summer time you will see crowd out up to very late.
You can Catch a Taxi at any time in front of those places to CDG.
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 4-50 minutes ( still check the traffic condition and rush hour commute time ).