The Piazza dei Miracoli is so much more than just the Leaning Tower. My husband and I spent over 6 hours enjoying the sites here. I'd recommend getting the combined ticket that covers the Camponile (the Leaning Tower, including the climb), Baptistry, Duomo, Camposanto, Museo delle Sinopie and the Museo dell 'Opera del Duomo. We were fascinated with the frescoes in the Camposanto and their corresponding sinopie in the museum. It seemed as if very few tourists actually ventured into any of the buildings, as the buildings were not at all crowded. I think the Pisa Cathedral is one of the most beautiful church exteriors in the world. If you go to the Baptistry, wait for one of the officials to sing a chord to demonstrate the perfect acoustics. This occurs every 15 minutes or so. (Or sing three notes yourself and listen to them form a chord.) The Baptistry is taller than the Camponile.
Many people don't recommend climbing the Leaning Tower, but we loved it. They limit the number of people admitted at each entry time, so it's not crowded at all.