Hello.
If you are looking for biking trails and tracks in Tuscany maybe I can suggest you this new website of mine, ridemontaione.com (I hope I am not transgressing any rule of this forum posting links, in case I beg pardon to administrators), where we have just tracked, described and took pictures of some amazing trails where you can explore this part of Tuscany.
I agree that if you are staying out of town, you should rent a car, moving from one little village to another one (which is a must in Tuscany) is harder without car, and with a car you can have the best frm your holiday.
I hope you' ll like it and have fun