Thanks for the additional info ;-)
Yes, I'll second spending the last night in Milan so you're already there... although if you've never been to Milan before, you could spend the whole entire last day there and go visit the Duomo, Leonardo da Vinci's Last Supper, the Castello Sforzesco (has lots of exhibits) and wander around Via Napoleone. I particularly really liked the Pinacoteca di Brera, if you're feeling like more art.
For Florence, as long as you stay anywhere within the ring around the historical center, you are within walking distance to all of the sights: take a look at http:
www.florenceaccommodation.com.
And as for the country feel without needing to rent a car - there are several towns along the main train lines but which are not generally very touristic or charming, with some exceptions of course. I'd recommend Certaldo, along the train line between Empoli and Siena, San Gimignano (you would need to catch train to Poggibonsi (or bus there from Florence) and then bus to San Gimignano. SG tends to be quite overrun throughout the day, as many tourist buses stop for an hour or so with groups. But it tends to get much quieter by dinner and after that. In May we should have nice weather to spend time outdoors... there are several B&Bs in farmhouses right outside the town within walking distance, so you can stay out of the town and just walk in to catch a bus to other places and then spend evenings there. Take a look at the recommendations found on this page:
http://discovertuscany.com/san-gimignano-accommodation/
In particular, the Niccolai places toward the end of the list are within walking distance to town (like less than 5 minutes outside of the town).
My special recommendation for a lovely town to stay in and not worry about car rental: SIENA. While it is a larger town, and it is spread out (the modern part), if you stay in the center, you will definitely not feel like you are in a large town but more like in a village. It just tends to be quiet and in May (not around the Palio), it should be really nice. We've stayed there for 3-4 nights on two separate occasions and it was really enjoyable.
Take a look of some of the offerings here:
http://www.tuscanyaccommodation.com/siena-city/
We stayed at the
Chiostro del Carmine and
Locanda di San Martino and I enjoyed this second one a lot, especially the views.