Fonni's territory is home to the very important Sanctuary of the Vergine dei Martiri, Fonni from the 17th century which is a destination for pilgrims right next to the Franciscans Convent. The church was built in 1708 in Baroque style and contains some curious paintings by local artists.

Another important church built in the 11th century is the Patron Saint church of San Giovanni Battista, located in the oldest section of the village also known as "Su Piggiu".

A little to the south of Fonni stood the Roman station of Sorabile, mentioned in the Antonine Itinerary as situated some 100 km from Caralis on the road to Olbia. Excavations made in 1879 and 1880 led to the discovery of the remains of this station, arranged round three sides of a courtyard some roo ft. square, including traces of baths and other buildings, and a massive embanking wall above them, some 150 ft. in length, to protect them from landslips, while a discharge certificate (tabula honestae missionis) of sailors who had served in the Ravenna's fleet was found in some ruins here or hereabouts. Near Fonni, too, are several menhirs (called pietre celtiche in the district) and other prehistoric remains like nuraghes.

The local costumes are extremely picturesque, and are well seen on the day of St John the Baptist, the patron saint. The men's costume is similar to that worn in the district generally; the linen trousers are long and black gaiters are worn. The women wear a white chemise; over that a very small corselet, and over that a red jacket with blue and black velvet facings. The skirt is brown above and red below, with a blue band between the two colours; it is accordion-pleated. Two identical skirts are often worn, one above the other. The unmarried girls wear white kerchiefs, the married women black.

Neighborhoods in Fonni are called "Rioni" of these the oldes is called "su piggiu" or the skin, probably derived by the fact the this is the highest and first layer of the village. Others include "puppuai" and "cresiedda" to the south, "logotza" to the east.