If all flights are on time you could walk out, get a taxi to take you to the city, drive around, have your group picture wherever you want and then drive you back to CDG. My guess is it will cost 150 euros or so.
If you come on the weekend, it is much better. During the week, traffic starts early and getting into and out of the city will cut down on the time you have.
You could also ask the driver to stop at a cafe for a few minutes; order a cafe au lait and a croissant.
By the way, there are also great breads and croissant at the airport but it is not quite the same.
I would not do this with the RER. Too many connections and stops.