I found Pisa to be "alright", the tower and surrounding buildings are fantastic and definitely worth visiting but the city itself was just a regular working town from what I saw. Did spend most of the afternoon watching Pisa play though, and their Ultras protesting against police.
Lucca is nearby and apparently stunning, been on our to do list for a while so I'm looking forward to watching New Order/Elbow there in July
Florence is brilliant though, 3 nights there won't do any harm at all, what a city. Train from Pisa into Florence is easy too and half the stops on the way make you go "Hang on I saw them on Football Italia!"