I wouldn't want to do it myself, with only 8 hours, but here's a suggestion. Take the RER to Gare du Nord. At the back of the station, there is a bus terminal. Take the No. 42 bus and get off at Champs de Mars and voila! there's the Tour Eiffel. But you'll need to take some of your precious time to buy bus tickets. Walking around Gare du Nord might not give you the right impression of Paris, by the way.
Please be aware that going through security at CDG after you have had your little Paris fling can be a lengthy process, so allow plenty of time for that. If your first flight lands in Paris behind schedule, or it takes extra long to get off the plane, you will need to rethink the whole idea.