You could look in the Rick Steves guide and see if there are recommendations for guides for Loire. You could also get a sense from the guidebook of what you'd like to see in Loire. Loire is a fairly large region.
I have traveled Loire Valley independently a few times. We did some wine tasting and have toured a lot of the chateaus. I have a couple ideas for you. You could take the train to Amboise and visit Clos Luce, which is daVinci's chateau and Chateau d'Amboise. You could do that independently. The town of Amboise is attractive for walking about. I'm not sure the train runs to Amboise every day of the week. When we were there about 7 years ago, it did not run on Sundays.
You could search Loire chateau to determine which chateaus you'd like to see. Some favorites are Chenonceau, Chambord and Villandry. Chenonceau and Villandry have beautiful gardens. There is a high speed train from Paris to Tours. I know there are a fair number of tours that leave from Tours. I can't recommend any specific tours.