I have got one extra night and day in Italy because of change in my return flight. I'm going to visit Venice, Verona, Bologna and Ravenna. I plan to leave Ravenna 6 July in the morning, and my return flight from Venice will leave 7 July at 3 pm. I travel by public transport. Any suggestion on where to spend the last day and night??

spend it in venice. Do anything you wish you had done. Pick up an extra souvenier. Take it easy and wander the city.

I agree with "Venice" but perhaps you might like to try staying on Lido or Murano if you don't get to these islands on your initial stop. The alilaguna to the airport stops at both these islands so you would have an easy exit.

Is Ferrara an option?

Chances are the trip from Ferrara back to Venice for your flight would involve at least one change of trains--before your figure how to reach the airport--and at least three hours. Check train availability for the morning of July 7. Personally, I wouldn't try this. Too much could go wrong.