Not strictly Tuscany (Liguria actually), but so many visitors to Tuscany have the Cinque Terre on their "must-see" list.
The Cinque Terre are five small fishing villages that lie down the coast from Portofino. Thay all cling to the sides of the steep coastline and were only reachable by boat until well into the 1960s. Nowadays there is a road that winds its way through cliffs, occasionally opening up into galleries that offer tatalising glimpses of the deep blue water bellow.