Paris day trips in March - advice needed

If you are interested in stately homes and palaces, then Fontainebleau is a nice day trip. Cant beat a combination of interesting history and a lack of crowds. Early March means the gardens wont be particularly appealing. But that's true for any garden around Paris at that time of year.

For an easy day trip, you might consider the Vaux le Vicomte. It's an easy train ride from central Paris. The gardens there are French formal, and as such don't have blooming flowers. So they're still spectacular, even in the off-season.

I went to Chantilly in October. We had a lovely, late-fall day, so we were able to enjoy walking through the gardens and lunching in the hamlet. We also thought the town was charming and had coffee there on our way to the chateau.

If it's a sunny day, Chantilly is beautiful, and there are the stables/horse museum and chateau to explore indoors, as well. There's an equestrian show that's also indoors, but we didn't go to it, so I can't comment on it.

Just had a quick look, and if I'm reading it correctly, it appears that the show isn't offered in March: https://www.chantilly-senlis-tourisme.com/en/the-destination/terre-de-cheval/les-spectacles-equestres/

The castle is charming, and there are multiple lovely views of it from the gardens. The gardens have a formal section, and then there are less tame sections, too. There are statues, ponds, a hamlet, a mini-menagerie, a forest, etc. to explore. You probably know this already, but Versailles was modelled after Chantilly.

In light rain, a visit to Chantilly could still be pleasant. However, on a day with heavier, more constant rain (not uncommon in March), I'd be inclined to skip it. We spent more time outdoors than in while we were there.

We were in Paris last March and enjoyed a day trip to Rouen with a family of 4. While not on your list, I think it's worth considering.

It's a straight shot on the train in about 90 minutes and the town feels like a different world from Paris. It's not close enough to the rest of Normandy to work in the D-Day beaches etc. but Rouen has plenty to entertain on a day trip itself (history including Joan of Arc connections, cool old town to explore, amazing cathedral, connections to Impressionist painters, Gros Horloge (clock tower), etc.).

I agree with the idea of Rouen. Other ideas include Reims and Orléans, both within about an hour train ride from Paris. You said you've been to Champagne, but if one of your stops there wasn't Reims, it's definitely worth a visit.