It would help if you'd clarify what "a day" is? Are you staying 2 nights in a CT village and so have 1 FULL day to explore the region or are you coming in for the day from another location, such as Florence?
It would also help to know what sort of sightseeing you intend to do? Not every visitor is a hiker so while some folks might spend a chunk of the day on the trails, you might just want to see a bit of each village.
In either case, I think it would be a stretch to give the 5 of them the attention them deserve in one day, even though they are very small and really don't have much that are "hidden". If you haven't read about the crowds during high season, that's a good idea so you know what you might expect.