Stopover in Paris

You don't need a booking.com offer for a taxi. The legislated flat-rate fee for a taxi from CDG to the Left Bank (where your hotel is) is 62€. Be sure to follow the lines/signs to the official taxi queue once you make it out to the open part of the airport.

I would imagine it would take longer than 36 minutes; I always estimate an hour. Although maybe at that time on a Sunday morning, 40 minutes is possible.

When going back to the airport, book your taxi in advance. That adds 7€ to the 62€ fee.

I agree to just get a taxi upon arrival. You will be badgered on your way to the official taxi rank, but just say "no." At the taxi rank confirm flat rate and that the driver takes credit cards.
You can reasonably walk to and around the Eiffel Tower from your hotel. You could also walk to Arc de Triumph along the Champs-Elysees. I would suggest taking the Batobus from its stop near the Eiffel Tower to its stop near Notre Dame for a relaxing and scenic transport option. Take the Metro or Batobus on the way back.

I assume that you will have been traveling all night and may be a bit tired. You have been fairly ambitious with the amount of walking for a first day after a red eye, but you know yourselves the best.

Since you are already in St Germain area for the food tour you might enjoy afterward the Delacroix studio and museum there. https://www.musee-delacroix.fr/en/museum-studio/the-museum-today/the-museum-today You can also see his work nearby in the Church of Saint Sulpice https://en.wikipedia.org/wiki/Saint-Sulpice,_Paris. Also from my notes I made for our own trip that week one of the Christmas markets is near SS on Boulevard St Germain.

Judy (from another Judy),
Based on distances from one place to another in your plans, I would say that you should probably spread these out over the whole day one and plot it out to avoid backtracking.
e.g. Eiffel Tower walk-around (it's closest to your hotel, then across Pont Alexandre III, to Arc de Triomphe, then walk down Champs Elysees toward the Place de la Concord). By now it is getting close to lunchtime, so maybe find someplace to relax and enjoy being in the city while eating. It is still a long walk (for me) to Notre Dame, so maybe metro or bus? (Personally, I find the Champs Elysees too touristy with lots of fast food places and souvenir shops, so I would take the metro to the Notre Dame area from the Arc. If so, when you exit the metro near ND there will be lots of cafes and restaurants on the left bank close to Notre Dame.) Lunch, then the cathedral. After that, a short walk to Ste. Chappelle (a must-see IMO). After that you could head back to your hotel area (Rue Cler) to wander a bit and seek out a place for your light dinner.
I recommend getting a Paris street map to see the locations of the places you mentioned to efficiently plan your sightseeing without backtracking.
Day 2 (a Monday) does have lots of museum closures, but there is Montmartre and the Basilica Sacre Coeur, the Opera Garnier, the Invalides (Army Museum with Napoleon's tomb), the Galeries Lafayette (Blvd. Hausmann location), the Jardins de Luxembourg....and the Louvre. Take your pick. Oh yes, take a one hour boat ride on the Seine on Vedettes de Pont Neuf. Inexpensive, narrated, one of the best bargains in Paris! Sit up top for the best views, but dress accordingly. It will be cold in December. We go every time in Paris.

Have fun!

"I understand the shops and cafes will close on Sunday afternoon and all day Monday."

This is not correct for the cafes in and around the Rue Cler area. I've not noticed it anywhere else either in areas where there are lots of tourists. I just got back from staying about 3 weeks at 2 different hotels in the neighborhood and the cafes are open every day. Right beside the front door to the Leveque is Le Petit Cler which is open ever day from breakfast thru the evening. Same with Cafe Central which is to the left from your hotel and Café Roussillon which is to the right, hahaha!! Some of the stores/fruit & veg stands may be closed but you won't be cooking anyway, lol.

I agree with the others about just getting a taxi from the taxi rank. Rue Cler is a pedestrian street but delivery trucks and taxis can drive on it so you will be dropped off at your front door.

I agree about just grabbing a taxi at the rank. The price is €65 though, to the Left Bank, just so you don’t think you’re being taken.

Don’t accept a ride from anyone who offers you one in the way to the taxi rank, even if they assure you they’re regular taxis. They’re not. Just get in the line at the taxi rank and you’ll be in a Taxi Parisien. No tip is expected.

Judy,
We just returned from our millionth time there. I always trot out the following suggestion for a meal: 'Le Temps de Cerise' (time of cherries) which is a coop-run restaurant in the funky-boho Buttes-aux-Cailles (hill of quails) district. That 13e place near Place d'Italie takes no reservations but remains one of Paris' best unpretentious spots for a home-made meal surrounded by locals.
Note that somehow, a more upscale, chi-chi eatery up in le Marais has gotten away with using the exact same name---not to be confused.
Good Luck.
I am done. the end

The Opera Garnier is quite spectacular. Next to it is the big department store Galeries Lafayette. Their Christmas decorations are also spectacular!

Thank you all for your great advice!! What a relief to find cafes will be open in Rue Cler area. And you even provided names!! I appreciate the suggested order for Paris Walk. I look forward to dining near Notre Dame! I just may try to make it to Montmatre and the Vendettes boat tour. And thanks for tip on taxi.
Thanks so much!!!

I'm happy the advice seems helpful. Those are my 3 favorite places to eat. A good friend always likes Oniwa, the Japanese restaurant just down in the next block when she wants a lighter meal. They are closed Sundays but open other nights. Amorino is essential for gelato, hahaha!

After you drop your bags, if you want to walk to the Eiffel Tower, I recommend this route for the biggest impact (and if the day is nice).

From your hotel, take a left out the front door and walk to the end of the street where it dead ends into Avenue de la Motte Picquet. Take a right and walk a couple of blocks to the open area called the Champ de Mars and walk toward the Eiffel Tower. On your left across the street is the Ecole Militaire or the Military Academy. This is where Napoleon went to school!! The google maps path won't let me walk down one of the middle paths (eye roll) but you will figure that out when you get there. From the ET, either backtrack on the Champ de Mars or walk out of the CdM area to Avenue de la Bourdonnais and come back to your hotel on Rue De Grenelle.

https://maps.app.goo.gl/h59HZTMPFf8bAHce9

On this route you'll also go by another cafe I enjoy, La Terrasse du 7eme right beside the Ecole Militaire metro stop on Ave de la Motte Picquet. It also is open every day!

Have a great time! I just spent 3 weeks in that area at 2 different hotels and generally eat dinner in the neighborhood!

Champs Elysees, Galeries Lafayette and Samaritane are magical at Christmas.

Hi Judy! We just (today!) came back from 2 nights in Paris, arriving on a Sunday morning leaving on a Tuesday morning. Lots of things open Sunday and Monday. In fact, I didn't really notice anything closed. All the Christmas stuff will be up by the time you arrive (they were setting up this weekend.) We spent our time around the 1st, 4th and 8th arrondissments so I don't have specific suggestions for Rue Cler area. But if you want to venture further afield, I'm happy to share with you a couple of good restaurants we ate at this time that were very, very good.