The Certosa del Galluzzo is a hill top monastery just outside of Florence - a short drive (out the Porta Romana and follow the Via Senese), or accessible by bus or taxi. The 37 bus used to service this route, but that route seems to have disappeared from the ATAF website - the 36 looks like the one to take now. You will have to walk up the hill (here is a large image of the route).
The monastery has a few monks left and tours are conducted about every half an hour. The collection includes some important works by Pontormo who came here to escape the plague in 1522. Closed for lunch from 11:30 to 3, and in the winter open only afternoons - it is probably best to call ahead for a visit: 055 204 9226