Sunday night layover in Paris (in December)

From a convenience standpoint, I'd get a hotel near Denfert-Rochereau. It is the terminus of the Orly Bus, and you can get the RER B there to CDG next day. You can get all over Paris with public transit to find where the Sunday night action is. That minimizes the time spent getting to and from your hotel, which for a short stay, can eat a lot of time.

I might stay in the Marais as its traditional Jewish history means that things often close there on Saturdays but are open on Sundays. It is also a lot more central and you can walk to Notre Dame (beautiful at night) and along the river. The Hotel de Ville also has an ice rink and other festivities at Christmas time. I'd probably, with one night, head over to Concord, ride the Grande Roue and walk up the Christmas Market to the Grand Palais, turn left and walk across Pont Alexandre (on the top of the hour to catch the Eiffel Tower sparkle) and then walk back up the Seine and either back to Concord for the metro or continue on up to Notre Dame for its beauty at night and then home from there.

It is also easy to connect with the RER B from Chatelet Les Halles or St. Michel. Rue Cler is a perfectly ordinary market street and as a rule market streets are closed down on Mondays. We did find that Rue Montorguiel has many open shops on Mondays though when we were there in October.

From Orly Airport get the Orlyval connection to Antony and then RER B (blue line) in to central Paris.
(This is also your line to CDG the next day). Look for hotels between Luxembourg and St Michel stops.
Lively St Germain and the Latin Quarter are at your disposal for Sunday night.

I've thought about staying near the Eiffel Tower or on Ile St. Louis for proximity to the sights and Champs Elysees lights or in the Marais because of its liveliness on Sundays. We definitely want to be "in the thick of things" but want easy access to RER at the same time. (Yes, we want it all, and at a low price!) We'll be winding down from a big wedding weekend with friends, so I don't imagine we'll be taking advantage of the nightlife much beyond a long stroll and perhaps a bit of mulled wine. The itinerary that janettravels44 laid out is exactly what I had in mind.

Thanks, all!

The Eiffel Tower is not even slightly 'in the thick of things' -- it is on the edge of Paris in what is a mostly residential and boring neighborhood and not near other sights and not near the RER B for the airport. In addition to the Marais, if you are planning on being out and about that evening, the Latin Quarter near St. Michel or Luxembourg RER stations is great.

I think it is fun to plan this sort of thing. We once arrived at our hotel near Victoria Station in London at about 5 pm with a flight out of Gatwick the next morning. We went to a pub for fish and chips and to a Gilbert and Sullivan performance at the Savoy and then walked across the bridge on the Thames by Parliament. All this was arranged on arrival. It was a very satisfying bite of London when we had only that one evening in transit from Rome. (the airline forced us to overnight but wouldn't allow us to make it two nights). So decide what a satisfying slice of Paris would be for you and relax and enjoy it. I gave my idea earlier but there are all sorts of things you might do -- just don't try to do too much. A hotel we love in the Latin Quarter with a great location for this sort of brief trip is Henri IV Rive Gauche.

Well reviewed 3 star near the Pantheon and 600m walk from Luxembourg RER.
http://www.hoteldesgrandshommes.com/en/