The village of San Felice Circeo is about 4 km away and has a good number of bars, restaurants, grocery stores, pharmacies, etc.

Terracina is about 12 km away, Sabaudia 10 km, the closest sandy beaches are located in the south of the Monte Circeo and are about 4 km away.

Monte Circeo is preceded by the stunning Pontine Islands, which are accessible by boat via San Felice in an hour. Anzio, the medieval city Sermoneta, the wonderful Ninfa gardens and the picturesque Sperlonga are all nearby.

Other points of interest around Casa Nettuno:
Rome (1.5 hours)
Naples (2 hours)
Positano (3 hours)
Abbazia di Fossanova (dated 1208)
Ischia and Capri (via boat)

Some useful links:

Parco Nazionale del Circeo
Isole Pontine
Costa amalfitana
Giardino di Ninfa