The Parco dei Mostri (Park of Monsters) of Bomarzo, a large park with gardens, buildings and fantastic sculptures, is a fun and different excursion, one that also inspired Salvador Dalì; meanwhile, the ancient village of Montecalvello develops around a castle where the famous Balthus, a contemporary painter, lived for thirty years.
http://www.italia.it/en/discover-italy/lazio/viterbo.html