My wife and I will be in Paris in September and would like to do a day trip to see castles.
We have already been to Chambord, Palace of Versailles and Mont Saint-Michel.
I see tours that go to Fontainebleau & Vaux le Vicomte Chateaux.
I would appreciate any suggestions on tour operators to use.

There's also Vincennes on the east side of Paris:

https://flic.kr/p/7CA2RP

"I see tours that go to Fontainebleau & Vaux le Vicomte Chateaux."

We've done that, and it was great. Here's our day with photos in first link below. You can use Paris City Vision (2nd link).

https://travelswithmaitaitom.com/cha...x-marveilleux/

https://www.pariscityvision.com/en/a...-fontainebleau

Thank you for your suggestion of Vincennes. We are staying in the 11th arrondissement and it looks like something we would like to visit.

Many years ago, but we took a day trip tour out of Paris that included Chenenceau.....very interesting.

A few years back we took Bus 46 from the 11th out to the Château de Vincennes. It's a longish ride but we enjoy passing through different neighborhoods. This bus route passes the Lion Fountain in the Place Félix-Eboué/Daumesnil, then the Palais de la Porte Dorée with its interesting façade.
Returning to the 11th, Bus 56 passes through the impressive and historic Place de la Nation. Here are the route maps:
paris_bus_46_map_hq_via_eutouring.pdf
paris_bus_56_map_hq_via_eutouring.pdf
https://www.eutouring.com/chateau_de..._eutouring.pdf

Of course, if you want to spend less time en route, you can take a speedy ride on Métro Line 1 directly to the château.

We have found the EUTouring site very useful during our trips. However, please note that there have been ongoing changes to bus routes since we have been in Paris in 2019, so please verify the accuracy of the information in these links.

Besides the Château de Vincennes, you can also take the RER A to the Château de Saint Germain-en-Laye west of Paris. It has a spectacular view over Paris (in the great distance) from the terrace of the château gardens. It is a wonderful picnic place.

I used Paris City Vision to go to Vaux le Vicomte, also, and they were excellent. That place isn't quite as easy to get to as some others, which is why I did that tour. I'm not even sure how you'd get there without a tour as the RER stop is quite a ways from the chateau. You don't really need a tour for Fontainebleau, that's easy to get to on your own. But if it's combined with the other, that's a good way to go. When I did it, you could do it only for Vaux-le-Vicomte in a half day.