What to do in 8 hour layover in Paris?

Take a taxi from the official taxi stand to the closest thing the Sacre Coeur
http://www.sacre-coeur-montmartre.com/english/
on the hill in Montmarte which would give you a view of the entire city.

This is the route
https://goo.gl/maps/KiivsXr1DYbPYd9HA

We took Le Bus into the city and had a nice one hour river cruise and dinner after, then took the bus back to our hotel at the airport.

I'm not aware of any sights near the airport, so sightseeing will involve going in to Paris itself.

Exactly what are the times of your arrival and your departure? That would color my recommendations. For instance, museums are close at 5 or 6 PM, so if your layover is not during the day, a museum visit is probably not possible.

Have you been to Paris before?

Is there anything specific in Paris you have your heart set on seeing? People here can then say if it's possible or not.

Take an uber straight to Trocadero and feast your eyes on the Eiffel tower then walk over to it, grab a snack in 7th arrondissement neighbood nearby. you can do in 3-4 hours.