For only two nights if you're coming and going from CDG, I would try for a hotel that's either very close to a RER station on the airport line or someplace that's easily walkable from one of the CDG bus stops. IIRC there's one at Gare de Lyon, and a thirty minute walk through the 5th to the 6th (in the direction of the Eiffel Tower, which is somewhat further in the 7th) is interesting and safe. This gets you to Luxembourg Gardens, the lively area of the Left Bank/Quartier Latin including the Pantheon and Sorbonne, a walk along the Seine and to Ile de la Cite (Norte Dame) and Ile St Louis (technically part of the 4th). You'll also find the Museum of the Arab World and (probably more than thirty minutes) the Rodin Museum and the modern art museum at the former train museum along the river in the 7th, almost across the river from the Louvre.

BTW I consider Champs Elyssees to be a tourist trap, although the lights around Christmas are pretty and fun to see once.

Have you checked rates for the Marriott Rive Gauche? It's good for airport transportation and walkable to the more interesting areas of the left bank, with some rooms having good views of Paris. The hotel itself is modern and somewhat oriented toward conventions, but having elite benefits of their lounge and room upgrades might make it worthwhile for you.