What to do on a long layover in Roissy-Charles de Gaulle airport? Of course you can window shop duty free designers or snag an overpriced mass produced croissant. Or...you can breeze through immigration, bust a move to the train station (in terminals 1 and 2) and head into Paris for the real thing! Other sites I found in my search recommended a minimum of 7 hours, but I didn't have that long, so here's my experiment, um, "experience". (-:
How I did it: landed @ 6:30 AM in terminal 2A on AA with connecting flight on El Al @ 11:10 AM (also in 2A). Overhead monitors @ Immigration said "7 minutes", but turned out to be 25...me thinks that it was coffee break time... No worries--signage was easy and the 8 minute walk to the train station was uncrowded and found an ATM on the way. At the station, ticket machines were intuitive and straightforward with a smiling assistant to help if needed. Be sure to get an RER ticket (not a TVG, unless you want to end up in another part of Europe) and although it doesn't say it during ticketing, the 10 Euros price is a roundtrip. Just hold onto your ticket in order to enter/exit, rinse and repeat.
Pro tip: Before your trip, Google the codes for express trains so that you'll understand which 4-letter code to target for the fastest trip into town. This matters. My first trip took 48 minutes and the return was 36. I debarked at St. Michel/Notre Dame...take the Notre Dame exit and you will eventually get on a one way escalator that takes you to daylight. Walk to the end of the street and turn left where you'll see Notre Dame. But wait...before you start snapping selfies, take note of an unmarked staircase about 10 yds from where you turned left. This is how you'll get back into the train station. A walk around the one block using a series of lefts is a great way to get a taste of Paris, cafes on the edge of the Quartier Latin and the Seine, but you won't find another entrance to the train station without much searching.
Heading back to CDG, I had to wait 8 minutes for a faster train. Immigration took about 15 minutes and security another 5. Flight ended up being delayed for more than an hour, so I had time to spare for more exploring. But that's a story for another time...bon voyage and happy exploring!