Day Trip from Paris? Suggestions please!

Depends on how early or late in April, and how early the spring flowers come into bloom (weather dependant). And, of course, how big a fan you are of gardens.

If the weather isn't conducive to a day spent outdoors, you might consider a trip out to the Chateau de Fontainebleau. Easily reached by train and the fare is included if you have the Navigo card.

Fontainebleau is an excellent suggestion especially if the weather isn't quite perfect. Fontainebleau is also on the Paris Museum Pass. You might also consider Rouen or Chantilly.

If you love walking around beautiful gardens, then Giverny is worth a trip. And in the spring, wow! And yes, if you have ever marveled at a Monet water lilies (or other) painting, it is marvelous to see the real thing. And of course, the very touristy photo opp on the made-famous-by-his-paintings bridge.

Provins. Walled, medieval town, upper and lower towns of different times. Train time was over an hour so lots of people don't think about it but we loved it. Here's my photos of it - https://andiamo.zenfolio.com/p960284446

I’m going to be a second vote for Fontainebleau or Fontainebleau in combination with Vaux le Vicomte. Vaux was actually one of the main reasons why Versailles was built (ie there was one realllly jealous Sun King) and is my favorite chateau in all of France. Fontainebleau is a wonderful hodgepodge of history and interior design and the audio guide is needed to fully appreciate the place. And the town of Fontainebleau is also a fun walk around.

Both are relatively easy access by public transport (train/bus), though we had the staff at Vaux call us a taxi back to the train station after finishing dinner there in an early May 2015 trip. If you do both, it just takes some more careful planning. Both are talked about in the RS book on how to reach individually and how to do in combo.

You asked about Giverny as well. We were there in very early May last year. The gardens are beautiful and we enjoyed our time there as a stop through on the way to Rouen, though I can’t speak to what the flowers look like in April. (I would look this up before deciding to do it) I think it helps if you’re a Monet fan because I was geeking out as we walked around, thinking about his paintings. But if I had to pick only one, I would see the chateaux.

I haven't been to Giverny but it's on my list for the next time in Paris (overdue!). Whether it's worthwhile for you depends on your interests, but it would certainly appeal to any lover of Impressionist art.

So far no one has listed the St-Denis basilica, in a northern suburb reachable by Metro, the first major example of French Gothic architecture and burial place of most of France's kings (or at least their hearts). As something of a Gothic junkie, I found it fascinating.

I don't necessarily think of St. Denis as a daytrip, since it's still on the metro but I really enjoyed the amazing number of French royalty buried in the crypt.

Provins sounds great. I'm sorry I haven't been yet.

Giverny is worth a trip, more so if you either love gardens or are a Monet fan enough to brave Versaille like crowds. If you go, it's worth planning to be there early or late because it attracts bus tours.

Fontainbleu and/or Vaux le Vicomte seems to be the other good choice that you haven't seen yet.

Giverny should be gorgeous in April with the bulb flowers (also be sure to visit the Luxembourg Gardens which is glorious with tulips in April). We put off going till about our 10th trip to Paris and were dubious about it as a tourist trap and had a wonderful visit; well worth doing; Giverny is near the end of this list of day trips i.e. not on the first page:
https://janettravels.wordpress.com/category/day-trips-from-paris/
I like medieval towns and so would encourage you to try Senlis; we also liked Provins but not as much and the medieval games are not on in April (nor were they in May when we visited). Other towns that were very picturesque were Moret sur Loing and Crecy la Chapelle. All 3 of these Senlis, Moret, Crecy are in the photo journal above so you can see what you like. Another town worth a visit is Auvers sur Oise where Van Gogh lived his last.

There are so many great chateau visits. You have done Versailles, but Vaux le Vicomte is also wonderful. And there is Fontainebleau, Sceaux, Ecouen, Malmaison, Maintenon.

And St. Denis is a great afternoon; it is on the 13 metro just outside Paris. We also explored the Bois Bologne and Vincennes last year -- there are lakes with rental rowboats and in Vincennes the ruins of an old World Fare human zoo i.e. pavillions where people from 'primitive colonial cultures' 'lived' as visitors toured the site.

I agree with others on Vaux le Vicomte and Fontainbleau. I enjoyed both of these a couple years ago via a 1 day van tour through Viator. This fall I am going to Malmaison which is on the edge of the city, but doable via Metro and bus. I want to see what was really Josephine and Napoleon's home. It would also be a good fit with Fontainbleau as this is where he said good bye to his troops and several rooms are as they were during his reign.