There are so many things you can see and visit here!
Let me give you a short list of the different sites, details and distances.
Monte Argentario (20 min.) with P.S.Stefano on one side and Porto Ercole on the other, characterized by their splendid inlets. www.monteargentario.it
The islands of Giglio and Giannutri which you can reach on mini cruises (20 min. + 60 min. by ferry). www.giglioinfo.it/
Maremma Natural Park where you can go trekking in the Mediterranean maquis (15 min.). www.parcoregionaledellamaremma.it/
Saturnia Thermal Baths famous thermal sulphur springs and falls which have a temperature of 37°C (50 min.). www.tuttosaturnia.com
The ancient Etruscan towns of the Tufo Rock Cities (Pitigliano, Sorano e Sovana) carved out of tufo rocks (50 min.). www.cittadeltufo.it
The ruins of the Town of Cosa in Ansedonia ad founded by the Romans in 273 BC (30 min.). www.ansedoniaonline.it/ita/cosa.asp
The Tarot Garden, Niki de Saint Phalle esoteric sculpture garden based on the Tarot cards (30 min.). www.nikidesaintphalle.com
The WWF Oasis of Orbetello with its 150 bird species (15 min.). www.wwf.it