Hi all - this forum provided me with useful tips when I was prepping for my recent trip to Paris, so I figured I'd repay the favor with adding some helpful, random bits of commentary and opinions based on my experience.
Museums in Paris - books all tickets in advance. You will thank yourself for doing this ahead of time when you show up on the plaza where lines form and realize you're able to save 20-40 minutes of your day because you don't have to buy a ticket there. Also, if you do buy timed tickets in advance, definitely book an early time for the Louvre (8:30-10am) because the museum will fill up throughout the day, and when you show up, look for the lines for reserved timed tickets; if you don't see them, ask an official looking person for help and they'll direct you to the correct queue.
Versailles Palace and Gardens - Gardens were definitely the highlight here, IMHO. If you've already seen the King Ludwig III castles in Germany, you might feel the same way. Same tip as above on the lines. I would also recommend setting aside an entire day for this trip and buying the all-inclusive 'passport' ticket, so you have access to all parts of the grounds for the day.
Rouen - You maximum need 7 hours here, which includes a 2-hour (slow) French lunch at a cafe. We did a day trip from Paris St. Lazare station and got in around 11 and left around 6pm. If you go, head to the Tourism office in town center and they'll give you a helpful paper map that indicates a walking route to see the best/most picturesque streets. If you are choosing between Rouen and Giverny or Chartres or Reim/Epernay for a day trip, I would maybe choose something other than Rouen... I just was really not impressed with the town.
Normandy WWII Tour / Bayeux - Absolute highlight of our trip and the single thing I'd recommend the most. We were able to find a French tour guide through Airbnb Experiences who did a full day trip and a decent price around the Bayeux area. We saw all the important sites, learned the history, and seeing the American cemetery on the Normandy cliffs was truly a very special experience. I'd also recommend a guide for this because it keeps you outside, learning about the sites and significant as you stand there, rather than spending the day(s) in and out of museums.
Other mentions:
Smoking - everyone in Paris is a "chimney," as my boyfriend would say. A huge amount of cigarette smoking and vaping, so if you're sensitive to that, beware.
Metro and RER B - we took the RER B from CDG Terminal 2 into Paris Gare du Nord and it was very easy and I would recommend. The Metro system in Paris is also very easy to use once you start using it, so I would also recommend that if you need to get around and don't have a car.
COVID Testing to Return to US - We stayed at the Marriott in the Porte de Versailles neighborhood. The reception folks there were able to recommend a nearby Pharmacie for us that was down the block. Upon visiting the Pharmacie the day before we left for the US, we simply asked the pharmacist for 2 antigen tests, needed to fill out basic name, rank, email and phone # info on a French Gov't website, then the pharmacist took us to the storage rooms upstairs, did nose swab tests, and the results were emailed and texted to us within 30 minutes - negative, thank goodness. From there, we were able to save the results to our iPhones as PDFs and uploaded to Delta App for the "Fly Ready" scheme they have at the moment, so you're pre-cleared to fly - note, this requirement to get a negative test might eventually go away.
CDG / Boarding Passes / Security - Because the wifi in CDG is spotty, I would recommend getting a printed boarding pass before going through passport control, because the Delta app on my phone stopped working as I got to security and I had to go find strong wifi to load it.
Thanks, and happy and safe travels to you all! Ps. not a lot of mask-wearing in FR.