Actually you wouldn’t have time to eat in Pisa, but if you want to discover this beautiful medeval town, while walking slowly slowly, you can have a lunch stop.
The best alternative should be pasta or pizza, of course with wine 🙂 because you are still in Tuscany.
This restaurant, Lo Sfizio, is the one I realized on our way to Pisa tower. For delicious penne arabiatas you pay €8. For 1 lasagna, 2 tortellini and 1 bottle of Chianti we paid €34. One thing to say, at most of the restaurants they use pork, not beef. If you have any sensitivity for this, ask what kind of meat will be served before you order.