Well, an 8 hour layover definitely gives you enough time to get out and see the city a bit. The amount of time that you actually have will depend on how long it takes you to disembark from the plane and get through passport control. You will also need to be keenly aware of what time you need to be back at the airport in order to allow enough time for check-in and security for your next flight. So, you’ll probably really only have about 3-4 hours max for checking out Paris.
Are you flying in to Charles de Gaulle? To get into Paris, you’ll have to either take the bus, taxi, or train. The train is probably your best bet. They run every 15 minutes and it takes 35 minutes to get to Paris. (So factor that in to how much time you have…) You’ll need some euros, so get those from an ATM. You can easily reach the Eiffel Tower or Notre Dame. You might also want to enjoy a French meal, since you’ll just be hopping back on a plane and be stuck with airplane food.
Above all, relax and enjoy your few hours in Paris. Don’t push yourself to see and do too much, and keep your eye on the clock!