we will be staying in Paris for several weeks and want to take a day tour in the loire valley, traveling from Paris by train. We want to see a few chateaus and do a wine tasting. Do you have a recommendation?
My recommendation would be to spend time at Fontainebleau, Chantilly, or Vaux le Vicomte, all easily accessible from Paris rather than traveling all the way to the Loire Valley for just one day.
However, if you really wanted to see châteaux in the Loire Valley on a one day excursion, I would take the early TGV from Montparnasse to Tours Centre (there is usually a direct train departing about 07h30), walk across the street to the Office de Tourisme and join one of their minibus tours. Take the TGV back to Paris in the early evening.
https://www.sncf-connect.com/
https://www.tours-tourisme.fr/en/preparez-votre-sejour/excursions-a-la-journee/
a belated thank you
If you are staying in Paris for several weeks, I strongly recommend spending the night in the Loire Valley. Amboise, Blois or Tours would fit the bill.