If you're going to visit Pisa, then this (of course) is probably the place you're looking to visit, and it's well worth it. I should imagine you will do what we both did and just stare at the Tower for a while wondering how it's still standing, and then join in with the masses of people pretending to hold it up. We didn't climb up it as my fiancée isn't the most keen when it comes to heights, but just seeing it from ground level was good enough.
I can highly recommend grabbing some lunch, and people-watching whilst chilling out in the sunshine on the lawn somewhere. Very entertaining.