Extra days in Paris

We did this in 2016. Research a tour company to pick you up at your hotel, take you to their location in Paris to join a bus group heading out to the beaches at Normandy. Go to Pont du Hoc, the museums, the cemetery, Omaha beach, etc. It is an amazing tour and time well spent. It will take most of the day however but well worth it. Sorry I can't recall the name of the company we used but it was reasonably priced and the bus was very comfortable. It may be a full bus rather than limited capacity like a RS tour. There are many such companies or you may prefer a smaller private tour to Normandy but they are much more money. The Normandy area is about 1.5 hours outside of Paris by bus.

Normandy is great, if you go visit Bayeux and see the amazing Bayeux Tapestry.

Another choice is to visit the Loire Valley with all its magnificent Chateux. We stayed in Blois.

I would stay in Paris! There is so much to see and do that you won't have covered. Plus your guide will have helped you feel really comfortable with getting around on the Metro so you can zip all over town. What do you like? Gardens? More museums? churches? WWII history? Walking tours?

If you want a day trip consider Giverny if you are a Monet fan or Chartres if you enjoy gothic cathedrals and stained glass.

To me, I'm not a fan of Normandy or Mont-Saint-Michel as a day trip - both are too far and too much to see to jam in a day.

When are you traveling?

I would definitely not take a one-day bus tour to invasion sites in Normandy. That's way too many hours on a bus and not enough time actually seeing the historical sites. If you decide you absolutely must do an invasion tour, I recommend considering the full-day Overlord tour designed for people staying in Paris. You take the train (on your own) up to Bayeux and are picked up by the Orverlord van right at the Bayeux train station. You are dropped off at the train station late in the afternoon, in time for the return train to Paris. The train should be considerably faster than a tour bus, and it will be more comfortable as well. This tour gives you almost as much sightseeing time as Overlord's full-day tour originating in central Bayeux, which is what most people spending time up in Normandy take.

I too recommend staying in Paris. One evening take a https://2cvparistour.com/ tour. If the weather is nice shop for a picnic lunch along Rue Cler on the Left Bank and Montorgueil on the Right. The latter is where the Stohrer (Paris oldest bakery) is located. There are other bakeries on this same block that are better. When you’re ready to prop yourself down and eat do so on the most western point that would be equivalent to a bow of a ship on Île de la Cité (bring a blanket). You won’t be the only one dining either. You'll see peope sitting on tablecloths with centerpieces, it's classy.
Another idea is to ride the metro west of Arc de Triomphe to the Grande Arche de la Defense. There’s a store there if you want to purchase goods for a picnic there too and you can sit on the steps and people watch.

Another vote for stay put in Paris. There is SO much to do and as you’ve said it’s at the end of a tour, some time to slow down and enjoy that magnificent city be it museums or time in a park or whatever else- it’ll be great fun. We spent a week in Paris and didn’t get to it all!

Relax in Paris, maybe see what the tour did not cover or return to something/some place you wanted to spend more time at.

Enjoy your trip.