Would Sarah Shuckburgh explore Amsterdam along a straight and narrow path or would she join her friend and leave no turn unstoned?

My plan for the weekend is as follows: Van Gogh, Vermeer, historic houses and - maybe - marijuana.

I don't smoke, I hardly drink, I have three grown-up children and I am a law-abiding, upstanding citizen. To me, a joint means a Sunday roast or an arthritic hip. But the recent plans to downgrade marijuana to a class C drug have got me thinking. I wonder whether the time has perhaps come to try it . . .

In Amsterdam marijuana is still technically illegal but coffee shops selling soft drugs are tolerated by the police, and business is booming. My friend Guy knows his way around, and on Saturday morning he steers me towards a neon-lit emporium displaying hundreds of mood-altering options. I feel at home with supermarket pick'n'mix sweet counters, but this array is baffling.

Jars of greyish green leaves, looking like dried oregano, sport exotic labels - Citral, Purple Haze, Super-Skunk, Chronic, Mary Jane, White Widow. Hard blocks of cannabis resin compete for shelf space with potent pollen, shaken from the tips of leaves. Hundreds of oddly-shaped pipes dangle from hooks. A whole wall is devoted to colourful cigarette papers and punters must also choose from dozens of roaches, or cardboard tips. The music is deafening. Teenage tourists, in baggy jeans and baseball caps, are getting as stoned as possible. They don't look very well.

"This weed is hype," enthuses the lad behind the counter, as Guy lights up. "Nowadays it's mostly grown hydroponically, here in Holland. There are tricks to growing it - they add liquorice or orange juice. Or urine . . . A cup of weed tea for you, madam?"

"Er, maybe later," I reply as I make for the door.

Back in the fresh air, we hire sit-up-and-beg bikes, and whizz through narrow cobbled alleys, over humped bridges, along tree-lined waterways, dappled with sunlight. At the stunning Van Gogh Museum, we shield ourselves from the crowds behind headphones, the English commentary creating the illusion that we are alone in front of each painting. Who needs drugs with such intense and vibrant paint?

At lunchtime, we pedal to bohemian Spui square, and sit at a pavement cafe among elegant intellectuals, our bikes chained with theirs to nearby railings. We choose Bittergarnitur - which the menu explains is "very many Dutch appetisers". Indigestible wedges of speckled Spam arrive, with lumps of pink and grey luncheon meat and tepid balls of mashed offal, garnished with a couple of damp gherkins and a heap of bland cheese cubes.

Guy wolfs it down. "Dope makes you hungry," he explains. "Now, how about a hash coffee or a hash cake?"

This second coffee shop is smaller and quieter and the clientele is older, more arty-looking. Latterday Oscar Wildes, Jack Kerouacs and Bob Dylans sit staring into space. Some gulp smoke through water-cooling bottles, or bongs.

"Dope heightens awareness and increases creativity," muses Guy, inhaling deeply. "Buddha smoked marijuana to achieve enlightenment."

But doesn't marijuana also make you giggle like an idiot and then become comatose? And why don't these customers look either creative or aware? "On our bikes," I say, bossily.

The enormous Rijks-museum presents a daunting challenge, with 5,000 paintings, a million prints and drawings, and 30,000 sculptures, but today we have a limited goal. Room 218 is empty apart from a uniformed guard, and we have the Vermeers to ourselves. One of us is stoned, the other is not, but it's hard to quantify our enjoyment. We both gaze in silence at the gentle, exquisite paintings, drinking in the detail, the light and shade, the stillness, the perfection. I am glad that I am not comatose or giggling idiotically.

That evening, Guy is alternately voluble and taciturn, but he tucks into a 21-dish Indonesian meal. The sky darkens as we stroll home beside inky canals, over bridges twinkling with fairy-lights, past gabled houses with glowing windows.

Until the 20th century, canals were so smelly that rooms at the front of these prosperous merchant houses were rarely used. Now, at every turn, our noses catch whiffs of cannabis. There seem to be coffee shops everywhere. Guy chooses one that heaves with rowdy men and groups of giggling girls. I study the menu, which includes hash pizzas, weed sandwiches, marijuana milkshakes, pollen tea and a choice of "smokes" as wide as a gourmet wine list.

Three young men in suits are inhaling dope through credit-card-sized cooling devices. A bearded man in sandals harangues me about the annual Weed Fair, where growers from all over the world gather in Amsterdam to exchange tips about equipment and techniques that will produce the strongest drug, and the maximum profit.

"Have a brownie and you'll be high for days," he promises. "No thank you," I say, politely.

Guy tries one. He tries another. That night he's sure he sees a flying saucer on the ceiling and he comments repeatedly on the amazing colours of the whitewashed hotel walls.

On Sunday morning, I am maddeningly alert and lively. "We have done Vermeer and Van Gogh," I say. "Now for historical houses."

Guy decides to stay in bed.

At the intriguing Willet-Holthuysen Museum on the Herengracht - the grandest of the grand canals - the 17th-century residents seem to have just popped out for a moment. Even less like a museum is the wonderful Van Loon house, on elegant Keizersgracht - another of Amsterdam's best addresses. I ring the doorbell and wander unaccompanied upstairs and down and into the gravelled garden.

I reach my third historic interior before it opens and wait in a busy canalside cafe. The Walletjes area is the oldest in Amsterdam - narrow houses of assorted heights line the canal - but it's hard to concentrate on architecture when riveting scenes are being played out at ground level.

As church carillons tinkle folksy chimes, I watch two chattering teenage girls fumble in their bags for keys and unlock a pair of glazed doors. An Asian woman carrying bulging shopping bags opens another door and greets a large black woman opening curtains at a nearby window. Behind her, a narrow bed, draped with a towel, looks as unromantic as a couch in a doctor's surgery. I realise that these ordinary-looking people are prostitutes arriving for the midday shift.

And yet it all feels very unthreatening as I sit alone at my cafe table. It is a bright, sunny day and middle-aged ladies pause on the bridge with their wicker shopping baskets, chatting with friends. Families cycle past with toddlers strapped into little seats. The cafes are full of people drinking coffee, reading the papers and watching passersby.

One o'clock strikes and the Amstelkring museum opens. Leaving the sex-workers, I climb up to another world - a cavernous church, ornate and beautiful, hidden in the attics of three houses. This clandestine chapel dates from the 17th and 18th centuries, when Catholic services were illegal - a time when theatres were considered immoral too.

Different eras bring very different moral codes, and nowhere have I seen a more striking contrast between the restrictions of 17th-century puritanism and the licence of today.

I stroll back through the sunshine. A few doors from our hotel, in the otherwise peaceful terrace of canal houses, a tiny coffee shop sits beneath the elm trees, and Bob Marley's face gazes from a window pane in autumnal Rastafarian colours. I find Guy installed at a pavement table.

"Natural Ethiopian bush is grown from the green earth and the orange sun," he explains in a slurred voice, "in contrast to unnatural laboratory-made weed."

I have done Vermeer, Van Gogh and historical houses. There is only one unfinished part of my weekend plan.

I am nervous. I order my usual drug of choice - a cup of tea - while I think about it. . .

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First published by the Telegraph.
©SarahShuckburgh