Seravezza, Castelnuovo, and Barga are all towns that exist in a lush, natural setting, providing easy access to innumerable local hikes that often include day treks between the small communes of the region. Barga may be the most beautiful, perched upon a hill and full of winding, climbing cobblestone. While there, be sure to take in the view from the Duomo di San Cristoforo at its high point for possibly the best perspective on the region.
Beyond the basic backpacking between villages, there are a few more adventurous options. Try the hike to the Orido di Botri, a spectacular limestone canyon replete with waterfalls and caverns, from Ponte a Gaio is a day trip for the truly adventurous, while the fortress hike from Castelnuovo to Monte Alfonso and the climb of Pania di Corfino (1,603 meters/5,259 feet) are essential if you've got a few days.
For more on this tucked away region of Tuscany, peruse our links; it may be the best set of adventure circumstances near Florence.