Trail of the Unexpected: South of Athens lie the Lord of the Sea's ancient remains

As you wander around the Greek capital, spare a thought for Poseidon. How different things might have been had the sea god won the epic showdown for the city, back in mythical times.

Competing for the right to be its patron deity, he and niece Athena agreed to conjure a gift and let residents choose their protector according to the offering they liked best. Poseidon struck the Acropolis rock with his trident; out sprang a fountain of flowing water. Athena summoned an olive tree, giving the city food, oil and shelter in one fell swoop. Poseidon's water on the other hand was salty and scarcely drinkable. It was a no-brainer for the judges.

And so today it is Athens that we visit, not Poseidonopolis. It is the Parthenon – the "world's most perfect poem in stone" built to honour the victorious virgin goddess – that dominates the skyline. But we shouldn't forget the city's second god. For when the heat of the city gets too much, the Lord of the Sea can provide the inspiration for a refreshing day's escape.

Any Poseidon adventure should start in the imposing neo-classical splendour of the National Archaeological Museum. It is home to an overwhelming array of impressive art, from human-sized vases to 3,000-year-old sculptures. To cut to the chase, scoot straight to Room 15, where, amid a deluge of white marble, your eyes are inexorably drawn to the majestic bronze statue of Poseidon.

It is the very definition of imposing: standing over two metres tall, his legs in full stride, his sculpted chest taut, his right arm pulled back as if about to unleash hell. The empty eye sockets only heighten the menace.

The statue dates from 460 BC. It languished at the bottom of the sea until 1928, when it was pulled up from the waters off Cape Artemision at the northern end of Greece's second largest island, Euboea. Some have claimed that, in fact, it depicts Zeus rather than Poseidon, but you can't help thinking that surely only the maritime god could have endured so many centuries underwater.

Having seen Poseidon in his urban incarnation, it's time to set off for his more natural environs. Athena may have her Acropolis in the heart of the historic town, but Poseidon can almost rival it. His temple lies about 70km out of town at Cape Sounion, perched on a dramatic cliff top with ocean panoramas. And the scenic coastal drive down Highway 9 to the southernmost tip of Attica offers many spots to stop off and sample the sparkling blue waters of the Aegean.

After navigating the crazy Athenian traffic, the first stop is Agios Kosmas. If you watched the sailing events in the 2004 Olympics, it may appear familiar: it's next door to where the crews launched their boats. The reason I had come here, though, was Christos Kortzidis, the mayor of this seaside suburb who, two years ago, went on a 24-day hunger strike to force the creation of a free beach for the people.

In a statute that Poseidon himself might have drafted, Greek law gives the public the right of access to the sea, but the mayor argued that privately run beach bars had hijacked all this section of coastline and therefore poorer families were being deprived of their constitutional rights.

Agios Kosmas is the fruit of that fight. As one of the handful of free beaches close to the city, it gets packed at weekends and the public maintenance leaves a little to be desired. But you have to applaud the fact that this pleasant bay is one of the few places where Greek families can lounge on the sand or splash in the water without paying for the privilege.

At the other end of the spectrum, and another 20km along the coast road at Vouliagmeni, lies Astir Beach. Here it's all white minimalism and curtained day-beds; waiter service to your umbrella-shaded lounger; and wooden boardwalks down to the sea to stop you scorching your feet on the sand. There's free aqua-aerobics or beach yoga classes for those not content to merely swim or sunbathe – and a beachside restaurant serves up balsamic strawberry salads.

Astir Beach even has

its very own Ancient Greek ruins. But if the €17 entrance fee makes your eyes water, head to Vouliagmeni's public beach, which has fantastic (and free) views along the bay. It's a narrow strip of sand and there's little shade, but if you bring your own umbrella, it's a great spot. Alternatively, compromise between the free-for-all public beach and the high frills of Astir by visiting the private Attica Vouliagmeni beach, where an €8 entrance fee grants you an umbrella, lounger and showers.

The final stretch of the drive is distinctly less stressful than the first few kilometres out of Athens. The traffic melts away and the road clings to the undulating coast, offering spectacular views of the Aegean as the sun begins its descent. All the while you crane your neck for that first view of the Sanctuary of Poseidon. When you finally make out its proud Doric columns silhouetted on the cliff, it does not disappoint.

Byron visited the place he called Sunium in 1810. He was so entranced that he carved his name on to one of the columns (although the authorities have now roped off the ruins so you have to make do with seeing his autograph in the guidebook). The Poseidon temple, which dates to the fifth century BC, is where mariners would come before they set sail, offering sacrifices to the maritime god so he would protect them on their voyage.

Getting there an hour or so before sunset is a magical experience. The waves lap at the foot of the rock 60m below as the shifting light plays on the marble. As Byron wrote in his poem, Don Juan: "Place me on Sunium's marbled steep / Where nothing, save the waves and I / May hear our mutual murmurs sweep..."

National Archaeological Museum, 44 Pattision Street, Athens (00 30 210 821 7724; odysseus.culture.gr). Open 8am-8pm daily (Mondays from 1.30pm); €7. Sanctuary of Poseidon, Sounion (00 30 22 920 39363; odysseus.culture.gr). Open daily 8.30am-8pm; €4. Greek National Tourist Office: 020-7495 9300; gnto.co.uk.