May is such a fabulous time to visit Tuscany! The colors, the perfumes and the activities. I definitely suggest that you look at staying perhaps 2 - 3 days in the city center of Florence so you can enjoy the "big city" atmosphere before heading into the countryside.
Especially pretty are the apartments at: http://www.florenceaccommodation.com...a-firenze.html
Once in the countryside you might want to look at the area around Montespertoli (check out: http://www.discovertuscany.com/tusca...ontalbino.html ) or Montaione and then for the second half of your time in Tuscany look more towards the south - Siena is a good option or even in the val d'Orcia landscape.