Anyone travelling to the south will travel through Tuscany and Sienna so worth stopping over for the historical and artistic sights: especially Florence/Firenze (stay outside the city near the train line: trains are fast, regular and cheap while car-driving is a nightmare).
On the natural side, the Maremma Natural Park in southern Tuscany is a must (stay on the coast near Grosetto - plenty of campsites and hotels ... and sea). The Park has lots of different habitats - umbrella pine woods, marais, sandy shore, cliffs, river, marsh, orchards, ancient farmland:
The area is so well preserved because some parts can only be reached when accompanied by an official guide in a small party. Well worth the effort.
Parco Regionale della Maremma