With 4 people, I'd use a taxi to and from the airport; you save so little by taking the train.
And then get carnet for around town as noted.
I would not spend a lot of time finding macarons and chocolates etc etc with only one day in Paris (obviously you do what YOU want to do but that would be my rec) You can get macarons in any bakery, they are not that fabulous and there are so many absolutely uniquely wonderful things you can ONLY do in Paris. Great chocolates are available anywhere but St. Denis, Ste Chapelle, the Louvre, the Orsay, Notre Dame (just walking around it is breathtaking), Pont Alexandre, the Eiffel Tower -- are ONLY in Paris. There are lots of Seine cruises; they are all pretty much alike. If you really want to do it you can do it when convenient near Pont d'Alma, Pont Neuf, the Eiffel Tower for example. I prefer walking in Paris along the Seine to being down in the ditch especially during the day time -- but the option is easy if you want to do it. Obviously you couldn't begin to do all these different things, but with so little time I would try to think what is the quintessential Paris experience for you. Maybe it is sitting in a tea shop eating macarons and if so, go for it.
The Christmas markets are fun at night with street food and music and kids rides and such, the merchandise is mostly overpriced junk. If you are there in the evening then walking up the Champs Elysees market then walking at the Grand Palais across the pont Alexandre gives a lovely view of several landmarks including the Eiffel Tower (we like to time it for the top of the hour when it sparkles) At Concord is the Grand Roue and it is a pretty spectacular view of Paris at night; I was reluctant but my nephews wanted to go and I was so glad we did. I great great pictures from the wheel.
If you are just there for one night another thought is to have a particular nice meal in a place with great ambience or view. Les Ombres has a great view of the Eiffel Tower through its glass ceiling. Bofinger is a beautiful room as is Train Bleu. Chez Dumonet, Josephine was a fun place for boeuf burguignon and grand marnier soufflets.
Have fun -- it will be hard to come up with a short list for 4 people with so little time but it will be fun narrowing it down.