The Campo dei Miracoli is the obvious draw and you should not skip it, but Pisa rewards anyone who walks ten minutes south to the rest of the old town. The riverbanks, the medieval lanes around Borgo Stretto, the small Romanesque churches, and the university quarter are the city most tourists never see.
Buy the combined Opera del Duomo ticket online before you arrive — the Leaning Tower is timed entry and routinely sells out a week ahead in summer. Everything else is bookable on the same site.