We rented bicycles and cruised the neighborhoods on both sides of the Arno. Enjoyed a gorgeous sunset view of the Ponte Vecchio while picnicking on the banks of the river.
Unless you are really running low on souvenirs emblazoned with male genitalia (and who isn't?) and/or non-descript leather bags, there's no reason to allow a lot of time here. It's a tourist trap and a dull one at that. Rub the nose of Il Porcellino for luck and kicks and be done with it.
My daughter and I serendipitously found ourselves at the top of the Spanish Steps the night Spain won the World Cup. There must have been 10,000 people there!
If you're hiring a car from Pisa airport, don't wait for the shuttle bus, it's only around the corner to the right. This will save you a big wait in the shuttle queue.