Best day trip from Paris for wine tour and castles

If we are talking just a day, it sounds as though you are with limited time.
I would forget the wine tour idea and read up on Paris restaurants, and try several over nice meals, over the whole stay. Versailles or Chantilly or Fontainebleau or Vaux le Vicomte are chateaux easy to reach from Paris.

The nearest wine producing area to Paris is Champagne. You should be able to find a guided day trip, including a visit to Reims cathedral.
Champagne is best known for its sparkling white wines, but they do also produce some red non-sparkling wines, which AFAIK are not exported.
The nearest area fro red wines is probably Burgundy, but that is too far for a day trip.

The Loire.
Under 2 hours you can hit a wine tasting coop and a castle/ chateau, back in Paris for a late night snack,

-S

Some Options:
Paris City Vision, Guided Visit of the Chateaux de la Loire and Wine Tasting: http://www.pariscityvision.com/en/loire-valley-chateaux-wine-tasting

Viator, Wine & Champagne Tasting in Paris: http://www.viator.com/tours/Paris/French-Wine-and-Champagne-Tasting-in-Paris/d479-3777MASTERCLASS

Yes, they do make red wine in the Loire, it's just not as well-known as the whites outside France. Partly because they often use a variety called Cabernet Franc that many people outside the region consider inferior. Try Saumur-Champigny, Chinon, or Bourgeuil.

It may be a bit far for a day trip, however.

There are several great wine bars, wine classes, wine and cheese tastings, and wine lunches and dinners in Paris; I would strongly suggest that you consider looking at those...Probably the most well-known is Ô-Château, but others are Musée du Vin, École du Vin, Paris By Mouth, Wine Tasting in Paris, etc. Frommers.com has an article titled "The Best Wine Tastings in Paris". It might whet your appetite and/or provide a great jumping-off point for finding other good ideas.

You can focus on wine only (that could be a bad idea!), or you can combine tastings with a food tour, too.

You can also make your tasting as simple or as in-depth/serious as you want.

While I understand your wanting to get out and maybe see some châteaux and wineries, there are FAR worse things than wine tasting in Paris ;-) There's plenty of atmosphere to go around there, plus you save several hours of travel time that you might rather spend IN Paris.

I think too that the Loire region is the obvious place for châteaux and wine. Duration highspeed train Paris – Tours: 1h15 plenty of tours are offered.

http://www.touraineloirevalley.co.uk/

Do wine tasting in Paris and devote one day separately to touring chateau in loire valley. Suggest you take train to Tours, then take Acco Dispo tour( small tours , vans) . look at site online.
It is not possible to tour châteaus without car( well I have heard of folks using bikes bit its not for those who do not like long rides and possible overnight)
Not sure how you think a small group doing either thing will her you near locals, even if you do it yourselves?