Cinque Terre

What time of year?
You can take the train to the last town along the Cinque Terre: Monterosso al mare.
Visit Monterosso then work your way back by train stopping at each town which are no more than 5 minutes to 10 minutes apart.
The last train getting you back to Florence in a reasonable amount of time is at 21:23 from La Spezia.

Our only C.T. visit (so far) was in June 2001, before there was a National Park, and much of the trails were closed off for maintenance. We stayed in Riomaggiore, used the train to head north to other villages over two days and nights. From Monterosso, we took a boat back south, viewing the coastline. So a combination of walking, hiking, train, and/or boat might give you a full chance to explore on your day, before heading back to Florence. Not sure of boat timetables or the operating months of the year.

For the trip from Florence to Cinque Terre, look at the 7:28am direct Freccia train from Florence Campo di Marte station (not SMN) to La Spezia Centrale. It takes just 1:43 and ticket prices can be very cheap if purchased in advance. The Campo di Marte is easy and fairly inexpensive to reach by taxi from central Florence.

Rick Steves' guide has little self-guided tours of each town. We found them quite interesting. I would select two of those that most appeal to you to do before lunch, then maybe another one or two if you have time. Travel between villages by train as others have suggested.

I normally prefer to stay in the Cinque Terre for a few days, but have also visited there on a day trip from Florence. You might consider something like this.....

• Train from Florence to Riomaggiore. The early train from Firenze Campo di Marte that was mentioned earlier would be a good choice. From La Spezia Centrale, the train to Riomaggiore is only about nine minutes.
• Explore Riomaggiore for an hour or so.
• Travel by train to Manarola (the train ride only takes a few minutes). Explore there for an hour or perhaps a bit longer.
• Travel by train to Corniglia. You'll need to take the Shuttle up to town from the station as it's a LONG walk if you take the stairs. As I recall, there's a small charge for the Shuttle unless you have a CT Treno card.
• Travel by train to Vernazza You might want to allocate a bit more time here, unless the place is packed solid with cruise ship hordes, in which case you may only want a short visit.
• Travel by train to Monterosso. Try to time your schedule to arrive there at about 17:00. Have a brief walkabout and then enjoy a fine meal at one of the excellent restaurants (Cantina di Miky is a big favourite with many here).
• Return to Firenze by train. There are departures on the current schedule at 19:07 and 21:07 (one change in Pisa) that will get you to Firenze SMN at 21:32 and 23:50 respectively (time about 2.5 hours). I took the latter departure and got back to Florence just before midnight.

There are many ways you could arrange the trip, but this will give you a brief glimpse of all the five towns and end the day with a fine meal with a beautiful ocean view as the sun sets.