Hi Peter,
I agree with other posters...do not rent a car for Cinque Terre. It's so easy to access by train. Why bother.
In regards to Tuscany it depends on where you're staying and what you really want to do. I was just there in May and my husband and I decided we didn't want to rent a car so found other ways to see Tuscany. One day we took the bus to Sienna & San Gimignano which was a great trip. However my favorite part was going to a wine tour at Castello di Verrazzano (http://www.verrazzano.com/en/index.php) . From Florence it was an approx. 30min bus ride into the Chianti region. Then you walk up through the vineyards to the winery. We did the Lunch tour (Chianti Tradition tour) and it was amazing. After an approx 1 tour of the place you sit on a terrace overlooking tuscany and have a lunch filled with meats, cheeses, and bread drizzled with olive oil. We sampled 4 wines, 1 balsamic vinaigrette, and 1 dessert wine. It was fabulous. And really I got the Tuscany feeling just from doing that and we took the bus the entire time.
Of course one day I'd like to go back and rent a car to experience more of Tuscany, but for the short time we were in Florence (3 nights) this was the perfect way to still experience it.
Let me know if you want anymore tips or have other questions. Have a great trip! :-)