As mentioned above, I always recommend doing the Fat Tire Bike Tour on First Day.
FAT TIRE BIKE TOUR - You knock out many of the bucket-list sites and you can say you saw them and not waste too much time doing it. The Bike Tour is fun too, getting to cruise through the streets of Paris with a large group makes it safer and is very enjoyable. Have Lunch or Dinner is an outdoor cafe outside the Louvre in Tuilleries Gardens is great. You can come back later and spend more time at the Eiffel Tower, Louvre and such. My kids were 8 and 11 the last time. 11 yr old boy had no problem riding with the group. 8 yr old daughter rode on a tandem behind me. She probably could have done it, but less worries at the time
DINNER CRUISE - The whole family really enjoyed the dinner cruise. Got to see all the sites, started during daylight and finished in the dark. Dinner was really good and beautiful views along the way. Highly recommend. We did a more formal dinner, but they have all types of options.