I'll admit that I'm the guy who's always telling posters that they should go places from JFK or LGA with similar stopovers and many here disagree with me, but I think it's definitely worth a try.

You'll basically have to play it by ear.

Since you're not waiting for bags, you can get out of the airport pretty quickly as long as there is not a long line for passport control and you don't get caught on a bus from a remote stand. Also, you should have a boarding pass and since Germany is inside Schengen, only need to be there one hour before your flight...

If you can get out fast the train (RER) is indeed about 45 mins (even though they claim it's 29 mins). Without bags, you can walk to the train in under 10 mins from just about anywhere in the airport. Don't get caught waiting for shuttle buses. Make sure to buy a return ticket to save a few minutes - about 8Euro each way if I recall correctly.

Take the train to the St. Michel station and when you get out look around and you will see Notre Dame Cathedral. Take a walk around inside, or just sit in a cafe and watch the world go by...

Sure beats sitting around CDG in my book....