Ciao Joann,
From your other post I gather you'll be in Siena and therefore will be seeking wineries in the Chianti area to the north of Siena or to the south in the Montalcino or Montepulciano area. But with a car you'll be free to move around further away so I'll offer suggestions for other wineries outside of those areas as well.
Many wineries offer tastings at a specific time on certain days which are open to anyone showing up, but many cater to groups and set up visits according to needs. Many others will set up the tasting with a phone call, so email them and see how flexible they are. Most are small producers so I am sure most will be flexible.
Not many know that the Chianti wine area extends all around Florence, while it no longer falls into the "classico" definition, it is still very good Chianti

.
You can try
Chianti Rufina wine at
Fattoria il Lago which also offers tours of their wine cellars:
http://www.fattoriaillago.com/cellars.php
They are located near Dicomano, to the east of Florence.
Fattoria Dianella offers Chianti and other red wines, as well as visits to the cellars, every Tuesday and Thursday at 3pm or on other days upon request:
http://www.villadianella.it/winetourstastings.php
They are located near Vinci, to the west of Florence.
Fattoria San Michele is near Cerbaia but also has 2 shops in Florence. Wine tastings are offered at all locations, the estate in Chianti offers the tour of the cellars as well. The addresses are on this page:
http://www.fattoriasanmichele.it/direct_resale.asp?lingua=en
Near Greve in Chianti in the heart of Chianti Classico area, we found this small family-run winery,
Montefioralle, that offers tastings:
http://www.aziendamontefioralle.com/EN/degustazioni.html
There are so many that it is hard to name all of them. One of the bigger names in the area is the
Antinori family, with several estates across Tuscany. A visit to their wine shop will allow you to taste a sampling of their wines from areas outside of Chianti (for example, Brunello di Montalcino and Bolgheri) as well as Tuscany.
You can visit some of Antinori's wine cellars at
Badia a Passignano near Tavarnelle Val di Pesa, where they also have a restaurant and shop. From the site:
"Every afternoon at 3:30 p.m. (except Thursdays and Sundays) it is possible to tour the ancient wine cellars, followed by a tasting of 4 Antinori wines. The visit lasts about two hours and we request a minimum of four people in each group. "
Their wine shop is also open for tastings - hours of operation: everyday (except Sundays) from 10:00 a.m. to 6:30 p.m. Find out more at:
http://www.osteriadipassignano.com
I hope this offers some ideas from which to start your search. The best suggestion might be to fit a visit a day into your plans as you move around the area on your vacation. Let us know if you need any more suggestions!
