If you really want to see another place, you can take an early train from Venice, spend the day in Verona or Padua (store luggage at the train station) and arrive in Florence in the evening.
However, you are giving yourselves just one full day in Venice. You land on Sunday morning, expect it to take 2-3 hours to get to your hotel to drop your bags. You'll probably be in a sleep=deprived, jetlagged haze. Monday all day when you are still adjusting after the overnight flight and time change. Tuesday you're already on your way to another place! You should spend at least 3 nights in Venice. Along the same lines, 3 nights in Florence gives you only two full days, possibly another half day on either side. You'll be rushed to see the sights in Florence if you want a (highly recommended) day trip to Siena. So I'd stay in Venice Tuesday and consider staying in Florence until Sunday.