So Many! :-)
On the western side:
-Siena is an absolute must and worth a good day.
-Volterra
-San Gimignano but very early in the morning or end of day around 4-5pm as there will be a lot less tourists
-Montepulciano is another great small town to visit. The old square, a couple of nice wineries, the old streets... Wonderful
-Pienza as well, very charming
-Orvieto will be around an hour away from you. I was absolutly charmed by the city and especially the cathedral.
-Montalcino especially to go on top of the fortezza for great views, visit a winery closeby and visit the Sant'Antimo abbey, 15 minutes away.
-If you have time and want to drive: Pisa will be 2 hours away and Lucca probably 1 hour 45 minutes.
On the other side:
-Arezzo of course
-Assisi is an absolute must as well. The basilica, the streets... Superb
-Gubbio is a wonderful village that hasn't change from medieval time
-Perugia is a bit bigger but very charming. The Piazza, the streets, the museum(national gallery I believe) is great.
-Cortona, a great looking village made famous by the movie Under a Tuscan sun