I stayed at the Westin in Florence for three nights and it was perfect. Great location. If there is any way to swing it, I would. I frankly think 5 days for just Florence is about right. I did three days and it felt like a forced march. There were still places I didn't get to.

We stayed at the Villa De Maschere outside of Florence for one night. It was a great change from all the cities. http://www.villalemaschere.it/eng/home.htm
We didn't have a reservation but got a small room. The food was very good.

You're young. I figure it would take me at least 6 months of solid travel to see all I want to of Italy.

I stayed at the Boscolo Venezia. It was OK. You may love it.