Can one rent a car for the day to see the sights? Are there tours, let say that go outside of Siena or Assisi?
Thanks so much.

of course you can rent a car for the day but you'd pay an outrageous rate for it. What kind of tours are you looking for? Wineries or what? Where will you be based?

I'm not so sure the rate would be outrageous but I rented a car for several days in Siena and it worked out ok, price wise. Most of the rental car companies have offices in Siena. I would book the rental in advance though, as they don't usually inventory many cars for walk up traffic.

the question was about renting for one day, which in Italy, is usually very steep.

Unlike larger cities like Rome, the smaller centres like Siena often have a minimum rental of 3 days. Renting for a few days is fairly reasonable, even for an automatic.

If you are planning on staying in both Siena and Assisi, why not rent the car in Siena and drive to Assisi, which is less than two hours away. That might help solve your dilemna.
Check autoeurope.com for rates and mapquest.com for directions if that appeals to you at all.