Paris to Loire Valley

TGV trains travel from Paris Montparnasse in about one hour to a station called “St. Pierre des Corps” which is near the town of Amboise. Amboise is where Leonardo Da Vinci was invited by the King of France to live what turned out to be his final 3 years in its Chteaau du Clos Luce. There are several towns near Amboise that have their own chateaux such as Chenonceaux and Langeais. A bus from Amboise reaches Chenonceaux in about 30 minutes.

Kenko,
With just one day, and no car, I would look for a full day tour that would take you to 2 (possibly 3) chateaux, depending on how early you get to St. Pierre de Corps station. Chenonceaux is usually considered to be the one must-see chateau (for good reason). Or perhaps you could visit Clos Luce and 2 chateaux. Regardless, your best bet for transportation outside of Amboise would be an organized tour. I would hazard a guess that there are more than one or two on offer. I cannot speak from experience as we always have a car, but seeing two chateaux in one day is very doable, and three is doable also (some need less time to see than others).

Going from town to town or chateau to chateau isn't a walkable option for a day trip. Bikes/cars, yes; feet, no. The Amboise advice is good, but if you can find a way to see Chenonceau, do so. It is lovely!
Bonne chance!

There are day trip tours that leave from the Tours tourist office which is across the street from the Tours train station. The tour I know of takes you to Chambord and Chenonceau and one other small family owned chateau.

I wouldn't bother with a tour that drives you from Tours to Chambord and return. That's two hours sitting in a minivan you don't really want just to visit the least interesting chateau in the Loire Valley.

I'd find one that visits Chenonceau and Amboise. That way you spend more time visiting chateaux than travelling.