Notre Dame Eternelle - Augmented Reality Show...happens in a theater underneath the plaza in front of Notre Dame (the Parvis) . Don't accidentally buy tickets to the version in the skyscraper district (La Defense).
https://www.eternellenotredame.com/en/
You have to wear a backpack with the equipment so careful if grandpa has shoulder issues.
The main palace at Versailles has a huge interior. You could still go but skip the gardens and biking. If you can get a bus tour/guided tour, it might help you keep out of the wet and maybe see some of the smaller onsite buildings like the Petit Trianon and Dairy.
We liked the Batobus Seine hop-on/hop-off cruise for sprinkling partly cloudy weather. We had no trouble with overcrowding.
On another rainy day, I spent part of the day at Galeries Lafayette where they have everything from Carhartt to Bonpoint (celeb high-priced kidswear). There is a tasteful souvenir store on the top floor along with some food and coffee kiosks.
You might schedule cooking classes. We loved the bakery tour/hands-on experience in the Marais that we booked through Get Your Guide. We made a totebag worth of bread and small cookies.