Day trip from Paris-suggestions????

I'm not sure Bordeaux is a great day trip idea: the train station there is about 30 minutes away from the interesting areas, same in Paris, and you can't get to the stations at the last minute because you would lose a lot of money if you miss the train.
So the journey is actually longer than 6 hours roundtrip, "all-in".

There are dozens of options nearer Paris (max 1 hour on the train), counterclockwise I can think about:
- Reims (cathedral, champagne cellars, great indoor market... but not much else for casual wandering)
- Lille (far more than a day's worth: old town, several excellent museums, very different vibe and architecture vs Paris)
- Senlis (medieval town), which can be combined with nearby Chantilly (chateau & park)
- Rouen (famous cathedral, good art museum, well-preserved medieval architecture)
- Chartres (often considered the "best" cathedral in France, and a cute and lively old town too). Can be combined with Rambouillet if you start early and don't mind packed days.
- Le Mans, which gets very little press but has an amazingly well-preserved late medieval town center (better than Rouen), with a cathedral that rivals Chartres
- Fontainebleau or Vaux le Vicomte
- Edited to add Troyes, mentioned below, which I forgot and which is great too!

And there's Versailles of course.

Thank you so much! Versailles is definitely on the list but I am going to also check in these other suggestions. Thank you again.

St. Denis for the first Gothic cathedral

We enjoyed Blois and Chambord: two chateaux in one easy. About an hour from Paris, nice town, two amazing chateaux. We did a nice bike ride to Chambord (about 45 mins) but you can also bus or cab.

Another vote for Reims as you can get there quickly by train. In addition to the cathedral, champagne cellars, and the market, there is also the Museum of the Surrender. That's where the papers ending WWII were signed. Versailles of course is where the papers ending WWI were signed. Very interesting to see both as they are quite different.

Over the years we've done a few day trips from Paris, all to cities I'd recommend. Rheims, Chartres, Rennes, Rouen, Dijon. Note that Rheims & Chartres are smaller cities so much is closed on Mondays. Rennes, Rouen, Dijon all have pedestrian-only downtowns.

Throughout the years I have visited most major cities north of Dijon in France, except Metz and my vote goes to Le Mans too, but my favorite remains (also much overlooked) Troyes. 1.5h with direct train from Paris.

Another vote for Troyes, a city with striking half-timbered architecture where I don't think I heard a single North American voice during my multi-night stay in summer 2017. Nor a bus tour, either. I suppose going for the least-foreign-touristy place may not be so critical this year, though, since the number of international visitors everywhere in France must be a lot lower than usual.

I wouldn't choose Bordeaux in this situation; as already noted, there are a lot of closer options that will be better for a day-trip.

Reims has quite a few sightseeing options and could be right for you, depending on your interests, but to me it's not an especially attractive city by French standards if you primarily want to wander around in a place that's not Paris.

There’s always Versailles (palace), Chartres (cathedral) and Giverny (Monet’s home and garden).

Very doable is Fontainebleau, the Chateau and the town.

We've done day trips not only for places in France, but also other countries. It makes for a long day (6 am - 11 pm), but is worth the trip if you have an opportunity. You can stay in France or go outside the country. We look at Paris as being our jump off point for a side trip - and always plan one. Sadly, we were suppose to go to Stausbourg once December, but he train strike stopped that trip.
We love taking the trains...it's a matter of how long you are willing to ride a train and how much time in the location you are visitng.
We map out our highlights and plan our time accordingly. We've done day trips to:
1. Strasbourg - quick train ride - the history of the town - the clock tower and the church are akk wonderful
2. Versailles - quick train ride and well worth it. Bring good walking shoes.
3. Giverney - quick train ride - again well worth it, but plan to go when the gardens are blooming. And walk the streets near Monet's house - there is a wonderful small church with a small memorial regarding British paratroopers.
4. Disney - if you just want to add it to you "I've been to Disney everywhere list" - otherwise, it's not a must to go spot - similar to other Disney locations in the US.
6. Bayeaux - did the Overlord half-day D-day beach tour
7. Malo - Mont Michel - Did a small tour (3 people for Malo and Mont Michel) - highly recommend small tour - 1 driver and car was found for about 80 Euro per person in 2019.

8. Geneva Switzerland - we had about 5 hours in city and the train ride in Switzerland was gorgeous. We saw the flower clock, Patek Philip Museum (beautiful watches), Cathedral of St Peter - walk up to the tower for a beautiful view of the Alps, and of course the swans and fountain of Lake Geneva.
9. Frankfurt Germany - longer train ride gave us 5 hours in the city. We love going for the German Christmas market
10. Brussels - we stayed close to the train and took their metro to visit the "old city" and enjoyed walking the streets for chocolate, lace, and waffles with plenty of time to see Manneken Pis
11. Amsterdam - again about 5 hours in the city. We prebooked Anne Frank's house - a must see. In April of 2019 - we actually took a bus from the city to see Keukenhof tulip gardens - this was a tight squeeze on the timeline but well worth it. Though in August, the gardens are not open, at least that is my understanding.
12. Karlsruhe Germany - beautiful zoo and botanical gardens.

Have fun and happy planning!

On the train (TER or TGV) , you have many choices, just depends on how extensive your interest in the place is, in which direction, what you aim to accomplish ,

By train from Paris I've done day trips over the years to Amiens, Meaux, Chateau-Thierry, Metz, Beauvais, St Germain-en-Laye, Bayeux, Caen, Strasbourg, aside from Versailles and Fontainebleau, Obviously, when I was motorised, there was more flexibility , ie,

to Compiegne, La Roche Guyon, Barbizon, Giverny, Vaux-le-Vicomte and Chantilly but still haven't made it to Soissons.