Day trips from Paris

https://community.ricksteves.com/travel-forum/france/recommended-side-trips-in-france-with-no-car

I'm sure Google and ChatGPT will give you great answers that are just as
opinionated as any that the forum members can provide.

Since you say nothing about your interests or budget, here are a couple:

Versailles
Fontainebleau
Reims
Strasbourg/Colmar

I'm sure there are many others, but that really means nothing given the
lack of detail in your query.

It looks like from a previous thread that you were planning on staying in Paris for an extended visit this Fall?

Would you consider spending a couple nights away from Paris during that time? That would open up the Normandy/DDay beaches and Mont Saint-Michel areas.

Are you interested in churches? Van Gogh? Chateaux?

Giverny is a day trip that is well worth doing

Fontainebleau, Chantilly, Chartres, Rambouillet, Malmaison... loads of possibilities.

My favorites:
Monet's House and Garden in Giverny
Chartres - the Cathedral and town
Chateau Chantilly - this is a great 3 in one sight. There is the chateau itself, an art collection in the chateau, and there are huge horse stables with a horse museum and live equestrian demos.
Chateau Vaux-le-Vicomte
Loire Valley Chateaux - it's long day, but you can do 2 or 3 Loire Valley chateau as a day trip. Do not miss Chenonceau!

Other good options:
Chateau Versailles - you can do this in a half day if you only tour the interior of the main chateau. If you want to see the gardens, Trianons, and Hameau, this is a full day.
Chateau Fontainebleau