Florence

Courtesy of the official Florence tourism agency: http://www.firenzeturismo.it/en/in-24-hours/visit-florence-in-24-hours.html

The fastest train runs take just over 2.0hrs each way from Venice to Florence. The earliest train arrives in Florence at 9:00am but others will begin arriving around or after 10:00am. Latest fast train departs at 21:30. At best, you have 11 hours. You easily have enough time to walk around il Duomo and the bapistry as well as walk across the Ponte Vecchio. This is free. If you have advance reservations to skip the lines, you can easily visit the Accademia to see the statue of David. Same with the Uffizi. Both would be difficult. Or, you have time to visit the Bargello museum or the il Duomo museum (treasures of il Duomo - great) or the Ferrigamo museum. You could have a fairly full day if you plan well.