Layover in Paris airport

You can sqeak in two, maybe two and a half hours, in town. You'll have to confine yourself to the area around Notre Dame, but that's okay. Ride the RER B commuter line from the terminal as far as the St Michel / Notre Dame station. When you pop out above ground the cathedral is just across the river. From the Pont Nuef on the west end of the island, you can look down the river for a pretty good view of the Eiffel Tower. Across the square from the main entrance to the cathedral is the stairs leading down to the archaeological crypt. Where you came out of the metro is the edge of the Latin Quarter, you might have time to wander there just a bit before heading back. The train ride is less than an hour each way and they're frequent - - every ten or fifteen minutes or so. Reverse the route to get back to the airport. Be aware that there are two trains that will share the same platform northbound - - make sure you get the right one. There are no luggage lockers at the airport, but there is a baggage check counter/room. You'll see the signs.

The logistics: If you don't have euros, get them at the first ATM you see in the airport. If the line is too long, violate every rule in the book and change a hundred bucks at the change bureau - - you don't have time to screw around, and cash works faster at the RER ticket machines. Getting all bills is fine, there's a bill changer next to the ticket machines. The ticket machine probably won't take your credit card, the manned booth will, but the lines will be long - - stick cash in the autoticket machine. Fifty bucks per person will be more than enough for the expedition (tickets are about $12/pp each way). Bathrooms: there are none in the stations. There's one down the stairs in the Notre Dame square - - behind the Charlmagne statue (dude on a horse). You'll need to tip the attendant a factional euro. The bridge behind the statue crosses the river to the Left (south)Bank where you first arrived, but further upstream. Across the bridge is a cafe that's not as bad as some of the others - - it says 'Panis' on the awning. They have restrooms that you can use if you buy a beer or cup of coffee. (Unless you spot street food somewhere, go hungry - - everything close to the river sucks). Sitting outside will cost more, but do it since it's your first time in Paris. There's another good spot for a drink on the corner as you work from the bird market to St Chapelle. If the lines are long, don't try for St Chap, you won't have time. Don't go off of the island onto the Right (north) Bank, you'll wander to far from the RER station and run out of time.

With a short amount of time Notre Dame and the islands (and a outdoor cafe) is a great way to get a feel for paris. Walk to the west end of the islands and look down the river, nice scene. I would be tempted to try for st Chapelle (judge the lines when you get there) the windows are glorious- even if you can only spend a quarter of an hour

The luggage storage facilities to which Ed referred is Bagages du Monde. They're located in Terminal 1 and in Terminal 2 near the TGV station. Single tickets on the RER and metro from CDG to anywhere in central Paris cost €8.70 each way. If you think you might take any metro rides while you're in Paris (rather than just walking around near Saint-Michel Notre-Dame), then you could buy one-day, zone 1-6 Paris Visiite passes. They cost €19.60.

Another option could be to go directly to the Eiffel Tower, visit it and go up to the second level so you can get a great view of Paris. Then walk across the street and down to the boat dock and take an hour cruise on the Seine - you'll see Notre Dame, The Louve, Orsay, and other great Paris landmarks. The metro is nearby. Check Rick's Paris book for transportation options from the airport.
Bon voyage!

From Nord to the closest rer/metro stop to the Eiffel Tower is fourty minutes. Then there's a ten minute walk to the tower. Assuming ten minutes (total) to make two train changes in each direction, that's an hour each way if you make no missteps. You will spend the bulk of those two hours underground and you will miss your plane.