I'd echo the choice of Bologna for a day trip.
Arrives BLQ in time for a nice lunch. Airport bus into city. Stroll around the attractive centre of town. Pick up some cheeses and back to the airport for a 18:30 departure.
I'm doing it in May, only problem I have is as a consquence I will miss seeing Liverpool complete the demolition of Chelsea in the CL semi final.
There is always Radio 5 for the drive home though..