Here's a great way for your family to get out into the Tuscan countryside for a day even if you don't want to rent a car. In 1996 the province of Siena decided to revive an otherwise dead secondary railway line that crosses the haunting crete senesi, the clay hills and broad valleys where the prized Brunello di Montalcino is produced. Using vintage locomotives and passenger cars, the Nature Train completes a circle between Siena, Asciano and Monte Antico several times a day in the spring and fall months.
Even if you're staying in Florence, you can take an early morning train to Siena and switch to the Trenonatura in Siena. The excursion train makes stops in many tiny hamlets, where you can get off if you like. A volunteer association has been marking paths near these stops so that passengers can alternate walking with riding. Some paths lead to farms where you can order simple homecooked lunches and purchase the renowned local sheep's cheese. There are also special prices for passengers at some restaurants near the local stations.
The Trenonatura trains leave from Siena at 8:30, 11:12 and 3:35. Full-day tickets are sold on board and cost 16 Euro for the full Siena-Siena ride or 11 Euro as far as Monte Antico (kids under 10 ride for free with their parents). You can also take your bike along for an additional 3 Euro. Ask for a map and a list of restaurants. Details and updated prices are online here.