8 hour layover at CDG

Where are you continuing on to? That will determine the time that you need to be back at CDG. It will take about an hour to get to Paris and an hour to get back. Where are you arriving at CDG from? Will your luggage be checked through to the next destination? Generally speaking, I would expect that you will spend between 1.5 and 2 hours hours for the roundtrip by mass transit and one hour from arrival at the gate to departure via train or car or bus to Paris. That uses up around 2.5 to 3 of your hours. If you need to be back 2 hours before departure, you will use up 4.5 of your 8 hours. So, I think that best case scenario, you are looking 3.5 hours in Paris and I think that it is more likely that you will have 2.5. I don't think that will work for lunch and sight seeing but you could probably see the Eiffel Tower and the Champs and Arc.

These days, even an 8 hour layover may not be enough time to leave the airport. But if your luggage is already checked through to a destination in the Schengen treaty Zone, it may be possible. First you must get a sense of how long it takes to get through immigration. Hopefully it is under 1 hour from the time you land, given that you should be back at the airport 2 hours before your connecting flight, and 1 hour each way transport time, that leaves 3 hours in Paris. Also consider what terminal you land at and what terminal you depart from.
1. Check carry-ons at the TGV airport rail station, http://www.aeroportsdeparis.fr/en/passengers/services/luggage/luggage-set
2. Buy a one day Zone 1-5 Mobilis card from a vending machine, to save time waiting in line at the ticket window. 16.60 euro per person.
3. Take the RER B train to St Michel-Notre Dame, cross the bridge to Ile de la Cite and check out the cathedral.
4. Cross the bridge to the Right Bank and take the No 1 Metro to Charles DeGaulle Etoile. Check out the Arc de Triomphe. If the line isn't too long, buy a ticket and climb to the top and check out the view. Champs Elysee, Eiffel Tower, Grand Arche La Defense, Basilica du Sacre Couer, Pantheon, Tour Montparnasse.
5. Take the No 2 line to Chappelle and walk the underground passage to Gare du Nord.
6. Get on the RER B back to the airport.
If some where along the way, you see a cafe that strikes your fancy, sit down a get a quick lunch.
That should be 3 hours worth.

Thanks so much for the tips. We are continuing on to Madrid so bags should be checked through.

I would just wander about the airport, looking at the stores. You might want to go outside and see the sun, stop a a cafe and get a coffee or a glass of wine and a snack. This might interest you too: http://www.tripadvisor.com/ShowTopic-g187147-i14-k1415362-Anything_to_see_near_Charles_de_Gaulle_Airport-Paris_Ile_de_France.html

There are a couple of cafes on the Champs Elysee. From there, it is only a little over a mile to the Eiffel Tower. Take a stroll. Check with your airline to make sure you can leave without repercussions. If they're fine with it, 2 to 3 hours in Paris is better than nothing!

"Check with your airline to make sure you can leave without repercussions"

Are you kidding? By the time they get to the airline desk to "make sure" it's OK, they will have lost much valuable time. You do not, as I am sure you realize, have 8 hours. Calculating travel time to and from the city at an hour each way, you will have at most 3 hours, time for a little look-around and not much else. If you take the RER to Gare du Nord you could pick up the hop on/ hop off bus right there for a little tour.

There is a baggage storage outfit called "baggage du monde" at CDG. If you google you can get its location. Bear in mind finding it and checking your bags and then retrieving them will eat into you time in Paris.

Personally, I would just stay at the airport but I have been to Paris many, many times so I understand your wanting to see what you can see. Just plan carefully.

Let's not forget the privilege of going through the security system on your return the CDG.