Doug, I would take RER from airport to Luxembourg and make your way back through Sorbonne, St Germain, and across the river to the Louvre eating and drinking your way through the cafes. I would then plan on an early dinner at Le Souffle; they are closed between 1630 and 1900 so would need to get there by 1600 or so); fantastic restaurant with soufflés available for each course. I would then walk up to Champs Elysses before getting the RER or taxi back to airport.
