Sounio is in Attica.
A bus from Athens costs €5 and some cents. The buses start on a terminal near the southeast corner of the Areos Park few blocks east from the Victoria Metro station (but there are more bus terminals in the area - ask in the KTEL kiosk). There is one bus per hour during a day. Buses also stop on some other places in Athens. The terminus is just below the temple. The last bus back to Athens starts at 9 PM in Sounio, late enough for the famous sunset over the temple. One way takes aprox. two hours.
Sunset over the temple. The temple area closes just after the sunset.
There is a taverna just below the temple, next to the bus stop. It is rather expensive and not very impressive.