Best Day Trip From Paris

I would vote for Giverny, especially if it's a nice day. I've been in early October before and was surprised at how colorful the garden was. Rouen is also good if you want to see the cathedral. Haven't been to Reims yet. Decisions, decisions!!

Chartres and Maintenon are two of my favorites. Rambouillet is also great. All accessible from Gare Montparnasse on the local TER

Branching further on the Versailles theme, you could go to Fontainebleau, or (if you're really into the Louis kings' history) to Rambouillet. http://www.rambouillet-tourism.com/

At the southeastern edge of Paris not far from Orly is Parc de Sceaux, which has a chateau and many acres of parkland.

You could to to Chartres and tour the Cathedral with the English-language audio guide.

The above are pretty easy by public transport. With a van/bus tour you could go to the nearest Loire Valley chateaux for a day.

Ah -- I see @Alexander posted some of the same ideas while I was writing mine. And yes I agree about Maintenon as well.

These are rewarding destinations, of similar effort to get to, that are very different from each other.

Pick one one(s) that call to you based on guidebook descriptions.

Or, say what you like to see and do, and the hive mind here will make suggestions.

Can only comment on Giverny: we enjoyed it a great deal. It pairs nicely with a visit to Musée de l'Orangerie in Paris where Monet's paintings reflect much of what you see "in real life" at Giverny. Our experience in May 2016: arrived 30 minutes before opening in Giverny to park and get oriented; there was already a (short) line at the entrance. Once in the gardens, we discovered that tour bus groups are ushered in thru a back entrance and the pond area was already filling up. All that in mind, we had a great visit...but arriving as early as possible certainly contributed to the enjoyment.

You can do Giverney in a morning, and then take a 1:00-ish train to Rouen and see that before returning to Paris. easy.

Reims is also easy, and very interesting, with the champagne houses and historic WWII sites.

Both trips are worthwhile, very accessible by train, and memorable.

Can only comment on Giverny: we enjoyed it a great deal. It pairs nicely with a visit to Musée de l'Orangerie in Paris where Monet's Water Lily paintings reflect much of what you see at Giverny. Our experience on a Sunday morning in May 2016: arrived (via rental car) in Giverny 30 minutes before opening to park and get oriented; there was already a (short) line at the entrance. Once in the gardens, we discovered that tour bus groups enter thru a back entrance and the pond area was already filling up. All that in mind, we had a great visit...but arriving as early as possible certainly contributed to the enjoyment. (Among the shops adjacent to the parking lot across from the entrance is a "carry out" window for coffee, pastries, etc.)

You get good ideas here. I think too Chartres with Maintenon and if possible together with Rambouillet is a good option if making use of the train.
Giverny with Rouen is a good idea too with the train. But in case you want to rent a car the Vexin Region north of Giverny, (nearby La Roche-Guyon worth a detour) is a nice area to drive trough. I think combined with a visit to Lyons-la-Fôret (also lovely countryside) and Château Gaillard (nearby Les Andelys is lovely too) with stunning view over the Seine. Rouen is not so easy to drive through, better place to visit with the train. A detour to Gerberoy is worth to consider but maybe too much for one day.

If you're a Van Gogh fan at all I would recommend a trip to Auvers sur Oise. It's an easy trip on the RER and free if you have a Navigo Decouverte card for zones 1-5. You get to see the room where Van Gogh spent the last days of his life, as well as the locations of several of his paintings. I felt like was walking in Van Gogh's footsteps when I was there.

Provins is also worthwhile and another "free" trip with a ND card.

Since these trips don't require advance train tickets you can wait to go on a nice weather day.