Ciao Aldo,
Sounds like a great plan! How many people and what period of the year? I think you have to decide whether you want to move from one place to the next or do some sort of ring rides where at least for a day or two you use one base before moving on. 
Also rent the bike here? Normal bike or e-bike? the e-bike might be a consideration, depending on whether you have the training yet or not - Tuscany has a lot of hills and some of them can be quite steep, even if short. 
Have you already considered some of these aspects of the biking tour?
You could start in Florence and one day head north into the Mugello, another day south into Chianti (to Greve in Chianti) - with returns each day to Florence. This just as a start to give you some food for thought. 
Giving you general direction to head to, not an actual destination. But once you're ready to leave Florence, for example, you could head to San Gimignano which is about 37 miles from Florence so you could decide at that point you move on from Florence. So maybe 2 days out of Florence, then move on to San Gimignano, then from there on another day head toward Siena (distance is 26 miles) or to Monteriggioni (16 miles).
There are some bike tour companies that plan these types of tours for you, as a group, with the bikes, van support, with luggage transport as well.... obviously, at a different price level than if you organize all the details yourself. With some planning, however, I do think it is doable. You will need good maps to also see changes in altitudes, so if you plan to take secondary roads (which are more fun anyways), at least you know if you will be including too many hills in the route.
I hope I've given you enough food for thought, let me know how else I can provide assistance!