Ciao!
Sounds like you already have a very good idea of what you plan to do and see both in Venice and in Tuscany.
You're right, since it will be the high season you should look into booking accommodation as soon as possible. During high season, many apartments out in the countryside book by the week and not by days so you will have to look into those that do offer shorter stays and hotels and B&Bs. In Tuscany there are many villas turned into small hotels or B&Bs that you should focus on. For example, how about this
villa in Chianti or this
small B&B in the center of the medieval hamlet of San Donato? If you would like the convenience of an apartment, I believe this
resort near San Gimignano might offer stays of less than a week but I am not positive.
You could look into having a
Tuscany cooking class at your place if you do get an apartment, it is really convenient having them come to you where you are.
Since you'll have a car, for wine tasting you could drive along the roads between Greve, Panzano and Radda and fine several wineries that offer tastings and tours. If you do a search for "chianti wine" on this forum, you'll find several thread that suggest specific wineries where you can also book meals along with the tastings and tours.
Definitely head to the area south of Siena called Crete Senesi and Valdorcia - magnificent landscapes and also wine region around Montalcino and Montepulciano. Many "enoteca" offer wine tastings and light lunches or dinners to go along with splendid local wines
