The Chao Phraya, Thailand’s main waterway, flows from Pak Nam Pho northeast through Bangkok to the Gulf of Thailand. Our two-night trip will take us upriver from the capital to its predecessor, Ayutthaya. It’s a trip that could take around 90 minutes by car, but we channel Siam the old-fashioned way and cruise at a calm six knots.

Bangkok, like Ayutthaya, was built for boats and this stretch of “The River of Kings” is a hodgepodge of factories, warehouses and Buddhist temples topped with elaborate snarling nagas (serpent-like guardian creatures) . Overloading temples is a recognized condition here, but our first stop, Wat Arun, is a must. Before the pandemic, annual visitor numbers were in the millions. In mid-March, when I was there, I hardly saw anyone there.

A huge psychedelic temple wedding cake, ornate tiers held aloft by legions of demons and deities, it’s quite impressive from afar. Closer inspection reveals that the decoration I had assumed to be painting is, surprisingly, mosaic. Like a bull in a china shop, it is encrusted with broken china.

“Chinese merchant ships used porcelain as ballast,” says our guide, Chana Ondej, who told us to call him Jack. “When they reached Bangkok, they threw it away. Same for the Chinese warrior statues here: all weighted. So King Rama III took it for the temple and his palace. In today’s throwaway society, the notion of a recycled 19th century king is certainly charming.

We moor for the first night near Wat Bot, under the gaze of an imposing golden image of a seated Buddha. We are just north of Bangkok, barely 45 km from our starting point, but already we have the impression of entering another era. The 401-year-old temple is closed for the evening and all is quiet except for the occasional barking of a street dog.

Meals on board are communal affairs, enjoyed in the airy, open-plan living/dining room, where walls of sliding windows allow everyone to have a view. Our talented chef moonlights from Loy Pela’s sister hotel, Anantara Riverside, where I spent several nights and took an inspiring cooking class under his guidance. The three-day menus offer new and royal Thai dishes, such as tamarind river prawns and an intense and spicy coconut and squid soup, as well as sorbets and palate-cleansing luxuries, including lobster and caviar.

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With Thai silk furnishings, antiques, hardwood floors, and hand-painted wallpaper in jewel tones, Loy River Song feels like a boutique hotel. This is a boat designed for refined relaxation, and quiet evenings easily transition from dinner to nightcaps on deck.

The ship’s four cabins – named after the river’s northern headwaters – follow an equally rich palette. Mine, Nan, has geometric paneling, louvered shutters, and jade-tasselled light handles, jazzy as clapper earrings. The mostly monochromatic herringbone bathroom adds to the art deco vibe.

I sleep deeply and wake up early. Bangkok behind us, the river widens and nature comes back to the fore. Dense deciduous forest stubble stretches along the banks, interrupted only by corrugated-roofed stilt houses and the occasional swanky new construction. While having a coffee on deck, I watch egrets fish for tangled water hyacinths.

Our next stop is Bang Pa-In Palace. Although its royal foundations date back to the 17th century, the surviving complex was largely created between 1872 and 1889 by King Rama V.

“Please dress politely” begs the flyer handed to us on arrival, whereupon surprisingly I arrive a cropper, thanks to short pants who have never failed an entrance exam so far . The palace remains an occasional royal residence, so there’s no room for my inadvertently irreverent calves. I am saved by a nearby stall which sells sarongs for a few pounds. Ditto for my equally chastened male travel companions, who, with no choice but the traditional Thai fisherman’s pants in fuchsia, rock a flamboyant vibe.

The palace is worth our sartorial sacrifices. A grand array of mansions, memorials, temples and towers, the bizarre architectural mix spans Baroque, Khmer, Gothic and Thai. The most impressive is the royal residence: Wehart Chamrun. A sprawling, double-eaved Chinese-style mansion with red, imperial yellow, and strips of gold leaf, it’s a mini-ringer for the Forbidden City.

At times, the estate looks like a madness. This is not the case for the second Siamese capital, Ayutthaya. Founded around 1350, until its annihilation by the Burmese in 1767, this historic city was one of the largest, wealthiest and most sophisticated in the world. Now a UNESCO World Heritage Site, the countless ruined stupas, pagodas and temples testify to the immense power that once existed here. Exploring the sprawling site on foot and long-tailed, Jack recounts secret tunnels and treasures, hidden centuries ago from the advancing Burmese army, that have yet to be uncovered. Turns out, killing your loyal slaves immediately after they’ve buried your valuables for you is a brutally effective way to ensure no one – and I mean no one – nicks your stuff. When Ayutthaya’s aristocrats were later massacred by the victorious Burmese, their secrets died with them.

We return to the ship to find the crew, now resplendent in their traditional Thai garb, waiting to welcome us with heartwarming late afternoon tea and sunset cocktails. As we board, an old man appears on the dock, chatting animatedly with Jack before rolling his eyes and leaving. “He doesn’t believe the boat is right for you,” Jack tells us. “He says it’s too big.”

Too big? It’s too late to protest but I want to tell him that after the sprawling palaces and temples of the River of Kings, the comfortable opulence of Loy River Song suits me just fine.

Abigail Flanagan was a guest of Loy Pela Voyages, which offers a two-night all-inclusive cruise for two from £4,300 (loypelavoyages.com). She was also a guest of the Four Seasons Bangkok at Chao Phraya River (double B&B from £424; fourseasons.com) and Anantara Riverside (double B&B from £110; cooking class and market tour from £95 per person; anantara.com)

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