How you define "highlights" depends on who you are. If you like to make sure you see the guidebook famous sights, then you need to spend at least 3 of your nights in Florence, and visit Pisa from there by train, and then maybe hire a guide to take you through the wine country, and drop you off in Siena. (or vice versa).

Please be aware that all the places I just mentioned will the mobbed with crowds of tourists and be extremely hot in August.

Other people would consider the "highlights" of a Tuscan visit having a rental car and taking drives through the Tuscan wine country southwest of Florence or south of Siena. Some people would want to book their hotel right in the middle of wine country, even right on a winery, with wonderful amenities like a pool or horseback riding, and a great restaurant.

Other people would define the "highlights" of Tuscany as visiting markets and the hilltowns were tourists don't go, following their own nose and throwing the guidebook away.

For other people, the "highlights" of Tuscany are seeing Lucca, San Gimignano and Volterra

The heat is inescapable, but the crowds can be avoided -- but not if you want to see what everybody who is following a guidebook or internet advice is being told are the "highlights."

I recommend Civita Di Bagnoregio. I went last year and it was stunning...

http://en.wikipedia.org/wiki/Civita_di_Bagnoregio

Are you flying into Naples? Rome? Florence?

Depending on your travel dates, you might be able to go to the Chianti festival if that interests you.

http://www.expochianticlassico.com/en/

If you want to go a short distance from Rome and see two gorgeous hill towns, I second mscleo's suggestion of Civita di Bagnoregio, and would also add Orvieto. We spent two nights in Orvieto (train from Rome Termini) and went to Civita after we picked up our car and headed to Tuscany. You can see my trip report from October to see where we went in Tuscany. The poster above does make the point that it's likely to be quite hot this time of year, so take that into account when making reservations (AC would be preferable). Have fun.