So, after hours of thinking and planning, reading and consulting including in this forum here are some thoughts and tips about my trip to Paris

Navigo Decouvert
I don't know why after more than 10 times in Paris I didn't explore this option. It can be bought from Friday to thursday and it covers monday to sunday midnight. 22.90 Eur for all 5 zones and additional 5 Eur to issue the card. Free use in the metro and bus. You have to have a passport picture. You can buy it in the ticket office of Il de France (not in the SNCF office). Why 5 zones? because it covers the ride from Charles de Gaule to Paris but also the way to Versailles and Fontainebleau (including the bus in Fontainebleau).

Fontainebleau
I've been there 6 years ago in the winter and I decidede to revisit now in the spring. the visit includes Naopleon's appartment, the Pope's Appartment and Roayl appartment. The Napoleon's and Pope's room are nice and the audio guide is very informative but it was a bit dull. The Royal Appartments are magnificent. Splendor at it's best. 3 additinal Eur to enter a chineese museum which you can easily skip. 2 1/2 rooms packed with oriental exhibits hardly interesting. If you do include it in your visit this museum is at the end of the corridor just before the staircase leading to the Naopleon's appartment ect. The gardens are vast and beautiful and if you heve enough time and you're not alone like me do spend some time strolling in the fresh air. Highly recomanded together with Versailles and Chantilly.

Mont St Michel
Friends here in Fodor's were not so enthusiastic about a day trip, certainly not on the last day of the trip but after many thoughts and hesitations I decided to do the long way on my own, and not an organised trip. There are few possibilities to get there, one is a train to Villedieu-les-Poêles for 27 Eur each direction. More popular itinerary is a train to Rennes which is twice more expansive but shorter in one hour and that is a big difference. I took this train and booked myself very comfortably in the first class to which I added just 2 Eur each direction. It takes 90 minutes only to Rennes. At the train staition a bus is waiting for you and 75 minutes ride to a dreary tourist's center out of which you have free shuttles to MSM itself. It was packed and though the shuttles are very frequent it is very demanded. Unless you push yourself expect to stand 10 minutes in the bus. Once in the island at the entance on your left and few meters further on your right there are public toiletts. They are little hidden and not marked very well and you can easily skip them. In that case you will only have toiletts in the abbey (or in one of the many restaurants of course). There were many visitors but not too crowded as I expected and the visit was lovely. Beside the abbey there are 3 little museums and a pass to all 3 is 9 Eur. Don't be tempted. Tiny rooms, shaby exhibits poorly presented and poorly explained. Total waste of money and time. The abbey is an impressive medival monument which means magnificent building work but no exhibits. The audio guide thus is imperative. The visit to the abbey takes no longer than 90 minutes. You can then walk on the ramparts. The view is exquisit, even more, I imagine, when it's tide time and the place becomes an island. Since it was low tide youngsters went into the sea, well, actually a disgusting and smelly mud.... The ramparts lead you through many restaurants the most famous is La Mère Poulard where you can eat an omlette for 45 Eur! I don't remember where I ate, maby La terasse. 25 Eur for 3 courses (cheese carpaccio, omlette and choclate mousse) and I felt i'm going to explode. After more window shopping of the many souvenir's stores I was ready to leave. It was 16:00 and the bus to Rennes left on 18:00 so I decided to walk all the way. it took me 40 minutes. Don't be intimidated, specially if you are travelling in company and not alone like me. It's an easy walk on a straight path without ups and downs. At this time of the year check you have a hat and a bottle of water since there is no shadow until you get to the same tourist's center where you have toilets but nothing to drink. Not to far I saw some restaurants or an Ice cream stand. S all in all, arriving at 11:30 and leaving the island on 16:00 it's 4 1/2 hours visit with almost 3 hours ride to each direction. Was it worth? well, yes. And it was so very simple to get there and back at a resonably hour (21:00 I was back in Paris). So to all hesitants: I recomand.

A quick visit and back to London
I took the Eurostar from Gare du Nord. The check in was open an hour before departure and the line was tedious and moved slowly which left me no more than 10 minutes for toilets and grab a sandwich and coffe. The next morning in St Pancras the check in was open hour and half before and the line moved smoothly and efficiently and I had plenty of time for nothing. Better that. Why London in the middle of a Paris trip? Because of the Van Gogh Exhibition at the Tate Britain. A MUST! Another must in Paris is the Emil Buhrle Collection at the Musee Maillol as well as the Courtauld Collection at the Fondation Louis Vuitton. Yes I'm a fan of impressionism and seeing Van Gogh in Maillol almost brought tears to my eyes literally.

Nice report zvi. I'm not sure I'd put myself through all that to do a day trip to Mont St Michel but you did show its doable as a day trip independently.

I'm really looking forward to the Van Gogh when I'm in London in July.

I am sure your information will be very useful to people wanting to do a day trip to Mont St Michel. Thank you for posting.

Few more words

Regarding Van Gogh, there is an exposition at the Atelier des Lumiers. It's a new place in Paris launched last spring with a 30-minute immersive audiovisual experience of Gustav Klimt’s paintings, featuring mural projections of the images set to classical music. It was breathtaking and was such a huge success they had to extend the display period. Now they are presenting their second project - a Van Gogh experience. After the great astonishment of the previous exhibition it was now somewhat predictable, the soundtrack less appealing, and following the great success much more crowded. Still it was beautiful and I enjoyied it alot. Highly recomended for children as well. Ends on 31/12/2019 https://www.atelier-lumieres.com/

Also recomended for children is the Tutankhamun exhibition in La Villette. A very expansive exhibition (you can't collect the tickets at the box office and they charge you for printing the tickets at home!) yet it is worth every cent. It is a unique opportunity to rediscover the history of the most famous of the Pharaohs before the permanent installation of the artifacts within the new Egyptian Grand Museum currently under construction. more than 150 original objects from the tombת more than 50 pieces presented for the first and last time out of Egypt. Don't miss it. Ends on 15/9/2019 https://expo-toutankhamon.fr/en/

Both exhibitions are very demanded. Book your tickets in advance.

Thanks for the report, I was thinking of including MSM in our Paris trip as well, although in December the days will be shorter....