I think that Pisa is a much maligned city, most tourists only come to see the Leaning Tower and Miracle Square, however there is so much more to see.
When walking towards the square from the railway station you will pass the H&M clothing store on your right, do stop and pop in. This was originally the stock exchange building and inside you will find the most beautiful frescoes and stained glass windows.
Continue and after crossing the River Arno is the statue to Garibaldi, who was instrumental in the unification of italy. Of course the Miracle Square needs to be seen, if you're fit, then climb to the top of the Leaning Tower, but do buy tickets in advance, the cathedral and baptistery are both a must see too.
Just a short walk is the old Jewish Quarter and the graveyard.
It is really worthwhile to take some time wandering around the little back streets, where you can find amazing little restaurants and cafes, serving wonderful and inexpensive food, primarily for the locals.
Piazza Dai Cavalieri is worth a visit this the university with wonderful buildings.
Along the River Arno you can find the tiny church of Santa Maria Della Spina.
Also the Royal Victoria Hotel, where Charles Dickens once stayed.
Although the city walls are no longer complete, there are sections where you can still walk along them and enjoy a view of the city below.
Pisa is worth a full day visit, there is a lot to see.