Day trips from Paris

Rambouillet, Chartres, Maintenon are all great

Rouen is interesting and a reasonably short train ride from Paris.

We went to Giverny on a day trip. It was very nice to be there in the month of May.

Just outside the “Périphérique” the Basilique Cathédrale of Saint-Denis, the final resting place of most kings and queens of France and the birthplace of Gothic art. And for many things having to do with aviation, the Air and Space Museum of Le Bourget. Further north of Paris historic Senlis and nearby Château de Chantilly. Have no idea if it is in easy reach with public transport from there, but a bit more west Auvers-sur-Oise for Van Gogh.

You can visit Giverny and Rouen with the train. But having a car you can also visit Giverny in combination with the Vexin Region with lovely countryside and places like La Roche-Guyon, Château Gaillard (stunning view over the Seine) with nearby Andelys, Lyons-la-Fôret and more north if time left Gerberoy. Rouen is best to avoid with a car, not so easy to find your way there.

Reims has a cathedral, a WWII museum and a basilica that all bear visiting, and of course, a couple of champagne cave tours.

For something very different, look at MiniFrance, easily accessible by bus from central Paris. You'll see beautifully landscaped miniatures of all the famous landmarks of the country.

There are dozens. Here are a few I have done in the last few years:
https://janettravels.wordpress.com/category/day-trips-from-paris/
In addition Auvers sur Oise, Chartres, Versailles, Chantilly, Rouen and Reims are all terrific day trips from Paris.
If you have a Navigo Decouverte, you can go for 'free' on the RER to any of these towns that are in the Ile de France (some of these are outside in Normandy e.g. Giverny and Rouen, but many of them are in the Ile de France.

And I would second Basilica St. Denis -- it is on the Paris Metro line 13 and is I think the most overlooked terrific tourist site in Paris (just over the border)

You could go a little further afield and take the TVG to Dijon. Journey is a little over 90 minutes. Very walkable city and the city centre is not far from the train station.

We just bought an English book titled "An hour from Paris" by Annabel Simms. Here's a quick link to her website so you can see this book. ( http://www.annabelsimms.com/ ). When we're there at the same time we plan to use her book and take 1-2 of her day-trips out to the countryside towns.

If you haven't been to Normandy, I would go there. To get the most out of your visit, I would probably take a day tour. There are a lot of tours that leave from a Paris. I took a Paris Vision tour that lasted about 12 hours and went to the WWII sites. If that doesn't interest you, they have other tours to Normandy.

Normandy is terrific but it isn't a day trip; I'd allocate at least 3 or 4 nights and preferably 5 or 6. -- We used the Annabel Simms book and have visited most of her recommended locations and enjoyed most of them. It is a great resource for Paris day trips that are slightly off the beaten path. She focuses on less well known sites e.g. Moret sur Loing rather than Auvers sur Oise or Crecy la Chapelle rather than Rouen or Chartres.

I second the suggestion of Giverny. We went on a cold, rainy June day and still thought it was fabulous. It was truly the highlight of our trip. The gardens are magical. Be sure to buy your tickets online ahead of time so that you can go in the back entrance and skip the ticket buying line.

In addition to Annabel Simms book, Ina Caro's Paris to the Past has wonderful information about the history of many places you can go to by train from Paris. With 6 months to plan I'd pick up both books and see which places sound interesting.

By train I have done as day trips from Paris...Chateau Thierry, Amiens, Caen, Bayeux, Versailles, Beauvais, Fontainebleau.... a lot of choices

Thank you Jean-Paul. Just ordered the book.