Monte Oliveto Maggiore is a wonderful suggestion. Magnificent frescos and herb gardens. That and Volterra, of all the smaller places in Tuscany, and I believe I've been to most of them, are the ones that are most memorable for me. But if I had to vote between Pienza and San Gimignano, it would be San Gimignano. Of course, my husband did most of the driving. Can't remember how far Lucca is from Florence, another possibility. Siena is also wonderful, but you may not want to visit it again.