Day trips from Paris

You can travel to and from Brussels in 1h 30m. The further out you buy your train tickets the cheaper it is. Furthermore, these trains sell out.
Both Bruges and Luxembourg take more than 2h and Bruges also requires a transfer. Europe is crowded during the summer so expect mass tourism wherever you go.
The two places to visit in Brussels are the Royal Museums of Fine Arts of Belgium then afterwards visit the Grand Place where you can find a spot to munch on mussels and French fries. Afterwards, find a chocolate shop so you have chocolate to eat on the train on your way back to Paris.

I'm a fan of both Luxembourg and Bruges, but I feel they are too far for a meaningful visit. How about Fountainbleu, Chantilly, Monet's Garden, or Reims (Champagne region)

There are a couple dozen great daytrips within an hour or so of Paris -- in such a short trip I would not waste time with hours on a train each way to visit another country.

Thank you for the answers!

I looked up all the options and Reims seems most exciting to me!

Any suggestions on what all to do there in a day trip? I was thinking of doing a cellar tour at Taittinger and explore the Notre Dame cathedral. Any other must do things, while we are there?

Thank you so much for making things easier for folks like us!

There is an excellent TI office right by the cathedral. They can give you a map with a walking tour of sights in the city. We didn't do Tattinger, but that sounds like a fun option. We asked the TI for suggestions of where to taste wine in town and Trésors de Champagne, by the train station, was their suggestion. Its coop type place with many small producers represented. You can buy a glass of wine or ask them to construct a tasting. We learned a lot and tasted some really nice (and expensive) champagnes at a good price.

Bostonfan,
In Reims there is also the Museum of the Rendition, where the surrender by Germany in WWII was signed. It's a nice museum with lots of interesting material. Also, fyi, the Cathedral in Reims is where most of the kings of France were crowned. While in Paris, if you go to the St. Denis Basilica Cathedral, you will find the tombs of many French kings. A visit to both of these places makes a nice bookended experience in French royal history!
Enjoy!

Jules, thank you for the T1 office and Tresores de Champagne suggestion. I will look that up!

Judy, the Museum of the Rendition is a must visit for me! I was not aware of the St Denis cathedral, that is such a cool bookending idea like you said, thank you!

Again, thank you all for making it so much easier for folks like me! :)

Rick lists the options in his guide. We did Strausborg and enjoyed it.

Bostonfan,
Just fyi, It is a walkable distance from the cathedral to the museum of the Rendition. I am old and had a bad knee when we were there, and even I could manage the walk. (We also walked to a nearby champagne house for a tour and tasting, but I can't remember the name.) It was a fine, full day, but not strenuous nor rushed.

Bon voyage!

Not far from Taittinger, which has an excellent cellar tour, is the basillica St. Remy which is very much worth a visit; you can see snapshots here.
https://janettravels.wordpress.com/2020/03/28/reims-cathedral-st-remy-and-taittinger/
There is a transport (tram? or bus?) stop right by the Basillica which will take you back to the Cathedral area which is also a good area to get flights of Champagne or lunch.

the TGV trip from Paris to Reims takes only 45 minutes and the earlier you book the cheaper that ticket is likely to be.

Judy, thank you! Your suggestions have been so helpful!

Janet, that link to your travels was very useful! I just booked the Taittinger tour and the TGV ticket, and will make sure to do the Basilica as well! Thank you for the tip!

St. Remy Basilica is lovely. Unfortunately, there was a service going on when we were there, but what we saw, yes, very much worth a visit.

Additionally, at that time of year, you will want to make appointments for winery tour/visits. (Except for Tressors)

It appears you have settled on Reims. If that is the case, you might want to eat lunch at Le Grand Cafe at 92 Pl. Drouet d'Erlon, 51100 Reims, France. It is well within walking distance of the cathedral, and not far from the train station. Do not cofuse it with the Hotel Le Grand which is across the street. You should be able to use Google Maps and locate it before you even leave home. The mussells were so good we ate there a second time.

Reims it is! We will do the cathedral, the Taittinger tour, the St Remy basilica and Tressors, if we get a chance. Thank you for the lunch suggestion at Le Grand Cafe! Will definitely go there!

I am really excited about this day trip to Reims. Thank you all for the lovely ideas and suggestions!