Hi! I'm glad you choose to (re)visit Italy!
I want to speak only of place I know well, so I may left some good spot behind...
If you plan to travel across Tuscany, you should stop at Monteriggioni. It's a little village still stuck in the Middle Ages, between San Gimignano and Siena. Leaving from Siena you should also stop at Orvieto, along your route, and at Civita di Bagnoregio, a bit away. Both are build on top of tufa rock and have rich historical attractions (the first) and natural beauty. The second has the best location cache I ever seen, from my point of view...
Around the Bolsena Lake there are a lot of other beautiful place, of them I don't spoke because my visit dates before mine "Geocaching era" But I suggest you to linger there and, if you have enough time, also around the Trasimeno lake. There I stayed at I Capricci di Merion. Cortona is another little jewel to visit and to eat at.
I hope to have given you some good point. Have a nice vacation!
P.S.: I live in Prato, near Florence.