Sounio is in Attica, the historic region of Athens city-state.
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 approx. 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.
Grecotel Cape SounioOn Cape Sounion near the Temple of Poseidon. It is generally far (~69 km) from the centre of Athens but the location is excellent. Bangalows with private pools.