I'm sitting on the foreskin of a whale. I'm not engaged in some bestial underwater tryst, you understand. I am perched on a stool in Ari's Bar on the super yacht, Christina O.

The whales' foreskins were made into seat covers by the yacht's late owner, the Greek shipping magnate Aristotle Onassis. It was a dubious joke but one that tickled the billionaire, who would jump at the chance of showing a female guest to a stool – Greta Garbo, Grace Kelly, Marilyn Monroe, whoever had swung by for drinks – and apparently declare: "Madame, you are sitting on the largest penis in the world."

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Those poor old whales didn't just offer up their foreskins for his pleasure, so to speak; they also donated their teeth for the stools' foot-rests and for the props on the counter (embellished with scrimshaw scenes from the Odyssey) that aided guests who had become a little the worse for wear at one of the starry parties Onassis threw in the Fifties and Sixties. A snuff box made from the bill of an albatross, displayed inside the glass bar, reveals an equal regard for bird life.

This floating homage to testosterone provides a fascinating insight into the character of one of the 20th century's most powerful men. Until now, just a few privileged people have had the chance to snoop around its cabins – since 2001 it has been available for private charter by anyone with a six-figures sum to fritter away on a week's holiday. But from May next year, a new overnight package, "The Maria Callas Experience", goes on sale at the rather more affordable price of €£1,500.

As the name of this new milestone journey reveals, Christina O isn't all about Onassis. The operatic diva – who had a torrid affair with the billionaire from 1959 until his marriage to Jackie Kennedy in 1968 – also haunts these decks. Thus, one highlight of this short voyage from Monte Carlo to the Bay of Cannes will be a recital of Callas's signature arias. A suitably stellar selection of Europe's bel canto sopranos has been enlisted to do the honours – Nelly Miricioiu, Claire Rutter and Mary Plazas – and they are to pause between songs to regale the melodramatic story of the lovers' doomed relationship.

First, however, I am to be shown around the yacht by the general manager, David Jeanjean. He points out that as well as the beak of that unfortunate albatross, the glass bar contains a diorama of ships set against a blue sea, which he thinks must have been some sort of game, perhaps connected to the defunct buttons on the front of the counter and the illustrated world map on the wall behind. Storm lanterns, whaling harpoons and a rope wound around the bottom of the bar confirm that Onassis loved all things to do with seafaring.

The ship is full of such curiosities to wonder at, but the best are on display on the promenade deck in the Callas Lounge, a large salon comfortably furnished with sofas and armchairs positioned in front of a small stage with a lapis lazuli balustrade where Callas once sang and where today's divas now perform. An astonishing collection of Callas memorabilia has been amassed from auctions by the current owners, a syndicate of Irish business people. A silver champagne bowl presented by La Scala; a publicity still of Callas in costume as Madama Butterfly from a 1955 production directed by Herbert von Karajan; a telegram, dated 19 December 1958, sent by the Aga Khan, complimenting another performance; all mementoes that confirm the diva's place in music's hall of fame.

But more interesting is the juicy stuff. A case on one wall holds the wedding rings of Callas and her cuckolded husband, Battista Meneghini. It's said Callas began her affair with Onassis on the yacht while visiting with her husband, and, in a row with Meneghini, she cast off her ring on the deck. On another wall, a short article about the rumoured affair between Callas and Onassis, written by a friend, the gossip columnist Elsa Maxwell, for the Journal American, sits alongside a letter from Maxwell to Callas apologising for the publication of the piece, explaining that she was required to write it by her editor.

Even more diverting is the ship itself, which was equipped and decorated under the watchful eye of Onassis. The wily billionaire bought the former Canadian warship, HMCS Stormont, for just $34,000 in 1953, while he was still married to Tina, the mother of his children, Alexander and Christina, after whom he renamed the ship. He proceeded to spend a staggering $4m on its transformation into the first and best super yacht. He gutted the 325ft-long frigate, added a third deck for his seaplane, and fitted the whole place out with the world's most luxurious furnishings, adding every convenience including, at one time, a hospital with an operating theatre, according to Nicholas Gage's informative book about the yacht and Onassis and Callas's affair, Greek Fire.

Onassis was in control of every opulent touch: he had a lapis lazuli fireplace built in one of the lounges, and a spiral staircase elegantly constructed with an onyx banister and brass balusters to connect the ship's three levels. Both remain for guests to see today. But the most notable feature is out on the main deck – a bronze-bordered swimming pool, which, when not in use, is covered with a colourful mosaic of a bull, inspired by the Minoan Palace of Knossos in Crete, which doubles as a dance floor.

The original eight guest state rooms on the main deck have been increased to 16 with the conversion of some former staff quarters in the bowels of the ship. For his own accommodation, Onassis had a suitably lavish four-room suite built on the bridge deck – now on offer for guests to stay in at a premium price. For his bedroom, he sent to Venice for the furniture, including Venetian mirrors mounted in ivory frames carved with shell motifs. For his bathroom, he chose Sienese marble and had a sunken bathtub lined with mosaic tiles and supplied with water through a gold-plated tap fashioned into the form of a dolphin. All these features have now been removed, though the onyx fireplace and Baccarat wall lights are still in place. And, to guarantee privacy, Onassis installed a secret door to the radio room to keep comings and goings unnoticed.

It's time to dress for cocktails and dinner, an eight-course tasting menu prepared by chefs from the three-Michelin-starred restaurant Don Alfonso 1890 on Italy's Sorrento coast, and it is paired with wines from its cellars by David Jeanjean, who is a sommelier by profession. No expense is spared for this procession of the finest food and wines – from the foie gras millefeuille, with a chutney of dried fruits, to the chocolate ingot for dessert. Its soporific effects would have sent me happily to my bed if it weren't for the promise of an hour-long recital by tonight's distinguished soprano, Majella Cullagh. (In a rethink the recital now precedes dinner to guard against anyone dropping off, which also means guests get to dine with the opera star.) She sings arias to our assembled group with passion, favourites including "Vissi d'arte" from Tosca and "Mercè, dilette amiche" from I Vespri Siciliani, underscoring the dramatic story of Callas's life.

And the evening isn't over. Back in Ari's Bar more drinks are served, and guests spill out on to the deck to take in the star-filled sky and the twinkling backdrop of the Riviera coast. In true Christina O style, this party looks like it will go on till dawn. And if I'm not careful, I may soon need a little help from those whales' teeth on the bar.

HOW TO GET THERE

The Maria Callas Experience on Christina O will take place in May and June 2010, sailing from Monte Carlo. The all-inclusive package starts at £1,500 per person and includes five-star cabin accommodation with breakfast served on deck, a tour of the yacht, a formal black-tie eight-course gourmet tasting dinner, all drinks, and a recital by a soprano of international repute – Nelly Miricioiu on 20 and 21 May, 3, 4 and 5 June; Claire Rutter on 22 and 29 May; and Mary Plazas on 27 and 28 May. Book through Abercrombie & Kent Private Travel at Harrods (020-7173 6440; mariacallasexperience.com).

FURTHER INFORMATION

Greek Fire: The Love Affair of Maria Callas and Aristotle Onassis, by Nicholas Gage (Sidgwick & Jackson).

MILESTONE JOURNEYS: Two more ways to wow your lover

Ride the Orient Express. The classic luxury rail journey takes guests from London to Venice in refurbished carriages dating from the Twenties and Thirties, with three restaurants, a bar, and 11 sleeping cars. The compartments are bijou to say the least, but beautiful in their detail. An overnight trip costs from £1,595 per person, including all meals but not drinks. Go to orient-express.com

Swoop beneath the rim of the Grand Canyon in a helicopter, for close-up views of astonishing multicoloured stone buttresses and columns that are more than 250 million years old. Fly out of Las Vegas with Heli USA and your half-day tour will also drop by The Grand Canyon Ranch where you can ride the range and meet real cowboys. From $419 per adult. Go to heliusa.com