I see a lot of questions about visiting the Cinque Terre on various travel forums and websites. This tour seems like the perfect answer: Cinque Terre Hiking Day Trip from Florence
For a limited time the trip is being offered for $100 a person. Trying to make it there yourself on the train for a day is pretty hectic - I have done it and didn't really have any time to see much once we got there. An excursion like this really makes it worth it if you have your heart set on seeing the Cinque Terre while in Florence.