Il tempio di Antas is a Punic-Roman temple dedicated to the god Sardus Pater Babai (Sid Addir to the Carthaginians). It is located south of the town of Fluminimaggiore. You can see it clearly on the map. To the south is the entrance gate and parking.
The link shows a picture of the map you find at the entrance to the site.
In the Valley of Antas you can hike along the ancient Roman road to the Grotte Su Mannu, an 8km long cave with interesting rock formations and artifacts from prehistoric cultures (payment required). There is a B&B nearby (with picture of one of the rock formations at Su Mannau).
There is also a Nuragic village and a Roman quarry to hike to (20 minutes to the quarry).
The site is well signposted along the S126 from Guspini to Inglesias. There is also an information office and small shop at the site.