I'm with Pieter on Siena and Firenze, but I would add in a few other stops. A nice little 500 mile jaunt stopping at Orvieto, Montepulciano,Cortona, Arezzo
Firenze, Lucca, Volterra, San Gimignano, Siena and home again. There are fabulous roads and passes throughout Tuscany plus San Gim is breathtaking.
Rusty