If you do this, the RER B train will continue a couple of stops past Gare du Nord into the middle of the city. I'd suggest getting off at St-Michel, right at the river opposite Notre Dame. If the weather's good, nearby is the dock for the Vedettes du Pont Neuf, a nice hour-long boat ride up and down the Seine, no need to reserve. It's also a good place to start a walk in any direction. Nearby attractions include Notre Dame (the west front), the Cluny Museum, Luxembourg Gardens, Pantheon, and neighborhoods like the Latin Quarter, St-Germain, Ile St-Louis, and Marais. The Louvre isn't far, but it's too big a project (including timed entry and security line) for your purposes.
But CJean's time warnings are, if anything, not conservative enough. You'll eat a lot of those nine hours in the logistics. CDG is not an easy airport even for people with experience there. And if you've been flying all night you won't be at your sharpest.