cabraddick,
I see that Mesland is only about 30-35 minutes to Tours. Tours is a city well worth exploring itself, and is in the center of the popular chateaux to visit. You will find proponents of each of the chateaux, but Chenonceau is by far the most recommended, and for good reason. If you could only see one chateau, it must be Chenonceau. IMO, others to see are Azay-le-Rideau and Chambord, for different reasons. A forum search will give you lots of valuable opinions, as some chateaux have magnificent gardens, others have excellent locations, etc. When last there, we went to Chenonceau in the morning (easy to wander for 2 hours, part of which is relaxing in the garden). We picnicked in a grassy area near the parking lot (picnic tables there), and then drove to Chambord to see the chateau there. IMO I would do two, or no more than three chateaux per day. Some are small and don't need as much time as the two I last visited. I would intersperse chateaux-visiting days with other activities so as not to have chateaux overload.
The tourist bureau in Tours or in other towns and cities of the area should have lists of vineyards to visit. Caveat...French wineries aren't usually drive-up, as in the US. Most require reservations and may have limited days and hours. Often it is best to go with a tour company/group. This way you can indulge in some tasting. French blood alcohol limits for driving are much stricter than in the US and it's not worth a citation. Of course, if you taste and spit, that is a good option too.
Mushroom caves are another place for a visit. I know there is one near Saumur, but there are others in the area, and will be a unique experience. Troglodyte caves and recreations of them would be another interesting activity. I don't hike so have no recs there, but hiking and cycling are popular in the area, so perhaps others will give you ideas.
Best of luck on your trip. Enjoy!