If the two of you have never been to Paris, your plane arrives early or is at least on time, and you make it through immigration and customs without too much delay, I'd say it's worth going into the city. The RER B train station is under the Sheraton. Fast, runs frequently, not expensive. Get off at the St.-Michel - Notre-Dame stop, walk around, have petit-déjeuner (croissant and French bread with butter and jam, coffee - more authentically French than brunch) at any sidewalk café, take the train back. Nothing against the CDG Sheraton, I've stayed there, it has a unique "ocean liner" architecture with the elevators in its "funnels" and a sea foam green color scheme, but Paris is Paris. An airport hotel ain't Paris.