Hi, May's a beautiful time to travel in Italy, lucky you.
Given that you love people-watching and natural scenery I would give some thought to staying in one place in Tuscany and exploring from there - there is something beautiful about going back to the same cafe for your morning Cappuccino every day for a week that will give you something that changing place all the time might not.
I would recommend Casole d'Elsa as a good central Tuscan village - and maybe somewhere like Orvieto - or even Tuoro or Perugia for Umbra/Tuscany.
In Central Tuscany, starting from the south, I would visit:
Pitigliano - Etruscan hilltop village carved on tufa outcrop - also has the oldest synagogue in - errm, don't quite know, but in the area for sure.
Abbadia San Salvatore - for the 9th century crypt built by the Lombards
Bagno Vignoni for the hot thermal springs - and Rocca d'Orcia or Castiglione d'Orcia too
Left to Montalcino - and coffee in the Fiaschetteria...
Below Siena cut west to Murlo - the old, little one. It has an Etruscan museum with a great figurine of an Etruscan with a cowboy hat.
Then Siena - plenty to see! - and on past Monteriggioni, west again to Casole d'Elsa, then past Poggibonsi to San Gimignano. Near San Gimignano visit Castelvecchio, an old village abandoned after the plague, 1348. On to Volterra - see the theatre and the Museo Guarnacci - up to Pisa, Lucca, Pietrasanta (for the sculptors) then up the coast towards Sarzana.
I'd stay in Lerici or Tellaro and go the Cinque Terre from there - they're crowded for my taste...
If in this itinerary you use Casole (or similar) as a base, nothing is over 2 hours away (except for the cinque terre) so you could set off each day and then return. And Florence of course - and Chianti....
Have a great time!