I would definately go into the city. You can get on the Le Car Rouge (that red on and off bus) and see an overview of the city in about 2 hours. Take the RER train into the city from the airport, get off at Notre Dame stop and you can catch the red bus off to the left side of Notre Dame. You would have time to take the boat ride down the Seine (discount tickets are available on the red bus), it takes an hour, great at sunset.
The train to/from the airport is very convienent. Have fun.