For your visit you need a few hours if you decide to only explore Piazza dei Miracoli. Extra time may be needed if you tour other attractions.
Worthy attractions: Piazza dei Miracoli complex, including the Leaning Tower and Duomo.
Left for another visit: Museo Nazionale di San Matteo; Santa Maria della Spina.
Last visit: August 2008.

On all our trips to Pisa, we limited ourselves to the magnificent Piazza dei Miracoli ♥♥♥ (also known as Campo dei Miracoli or Piazza del Duomo). But even that one piazza more than justifies a trip to the city.
The piazza is, of course, the setting of the iconic Torre Pendente ♥♥♥. Hordes of tourists photographing themselves in “propping-up-the-tower” poses notwithstanding, it is an amazing sight, both close up and from the distance, when the tower appears to be peeking out from behind the duomo.
Small groups of people at half-hour intervals are allowed to climb to the top. For those expecting a sideways climbing adventure, you may be disappointed: The steps run largely inside the tower and the tilt is entirely non-discernible. The view from the top is great in respect to the piazza below; the rest of the city is not really inspiring when viewed from above. Advance ticket purchase is pretty much the only way to get in during peak season. Note: Children under 8 are not allowed to climb.
The tower stands next to the Duomo ♥♥, with its wonderful four-tiered façade with creamy colonnades and blind arcades. The duomo is uncharacteristically bright inside due to large windows on its upper levels. On our visit in 2003, we were not allowed to wander around, but instead observed the interior from a roped-off area near the entrance (we did not go in on our more recent trips).
The other major building on the green field is the baptistery ♥, which is both imposing and graceful. The look at the interior is worth the entry fee for those who enjoy Gothic and Romanesque lines.
The fourth element of Piazza dei Miracoli is the Camposanto (cemetery), which we never visited.
The city has a couple of museums and interesting churches to offer in addition to the piazza, and I can hazard a guess that they are never crowded, but I do not know anyone who budgets time for those.
Car parking
It is possible to find paid parking on the streets not very far from the piazza even in the peak season. I prefer to park at one of the big lots across Via Carlo Salomone Cammeo from the piazza, between Via Andrea Pisano and Via Vecchia Barbaricina.
Places to Eat
Singularly the worst culinary experience in Tuscany can be obtained by having a meal at one of the cafés on the pedestrian alley leading to the right from piazza’s Porta Santa Maria. The food is of the frozen/re-heated variety, raising clear concerns about its freshness. We only tried one such place, but their appearance suggests that they are all similar.