In the crook of the Italian knee is a green and hilly country full of treasures yet to be discovered by the mass devotees of neighbouring Tuscany and Umbria. On my return from a dazzling week in Marche, I feel more than usually ambivalent about the travel writer's role as potential despoiler of the fragile idyll described. "You are my worst enemy," said a stranger recently to the Marche tourism representative in the UK. "I've been holidaying there in perfect peace for 15 years - and now you are trying to ruin it."

Bounded by Urbino and Ascoli Piceno, by the Apennines and the Adriatic, Marche is a hill country, bisected east to west by innumerable rivers. Any north-south journey becomes a bit like an ant's attempt to cross a ploughed field, each crest revealing prospects so seductive that there seems little point in ever making the effort to move on.

Inland, Marche comes straight from the pages of a well illustrated children's book: set against a mountain backdrop, a rumpled quilt of a landscape patched everywhere with green, its vines stitched in straight lines, its white roads ribboning and vanishing between a series of gently rounded hills, with toy farms tucked into every fold.

It takes less than a day here to forget any notion that, for sightseeing, Marche might be a poor relation. Neighbouring Tuscany and Umbria may have scooped the jackpot, but neither region has Marche's green peace nor its profusion of pretty, fortified towns. Almost every summit seems to be crowned with a near-perfect set of apricot-coloured walls and bell-hung towers, each town begging merely to be idled in. Day after day, our carefully planned itineraries collapsed as we failed to resist the siren call of coffee in yet another sun-baked square.

Other comparisons are inevitable. Tourism is negligible here, the roads blessedly empty, the food cheaper. The region has the lowest crime rate in Italy. Few British tour operators have taken any notice, although there are sinister advance signs of German house-buying. Urbino, the nearest thing to a honeypot in the region, has only eight hotels. In the course of a late spring week we heard no English voice, bar those of our neighbours in the "Vacanze in Italia" flat below. Not one; it pays to speak a bit of Italian here.

Marche is not entirely free of tourism. The long strip of gridplan Adriatic resorts, a southern extension of Rimini, are popular with the home market. They did not look their best on a chill spring afternoon, mainly spent negotiating the parallel factory developments that line much of the coast. But we did like the look of the tiny resorts on either side of Monte Conero; Portonovo, at the foot of plunging cliffs, and Numana with its rows of blue and yellow beach huts, freshly painted and ready for the well-behaved summer denizens of Sirolo, the classy resort on the hill above.

At Porto Recanati we found Ristorante Torcoletto and one of the best fish meals it has ever been my good fortune to demolish. Beatrice Crostelli cooks with inspirational unfussiness; gnocchi with lobster in a wonderful fish stock, marinated raw salmon stuffed with sea bass, crayfish tails on rocket with tomato and basil. The fish from this part of the Adriatic are considered so superior, says the proprietor, Fabio Crostelli, that moves are afoot to give them an appellation in the same way as wine.

Lack of outside pressures has kept Marche cuisine faithful to its roots. We were based in Montecarotto, where our hosts, new to the holiday villa scene, were concerned that a Saturday arrival might have left us no time for shopping, so they invited us to a sumptuous Sunday lunch. Thanks for Anna Carotti's meal were waved aside. Cresci, the beautiful flat bread cooked with oil and rosemary, home-made pasta, home-reared goose and rabbit and home-produced apple cake, all washed down with the farm's own Verdicchio wines, were "nothing special".

We wondered, ruefully, how long the apparently universal niceness of the Marchegiani would survive a tourist onslaught. The same sort of meal, very reasonably priced, is available in modest restaurants throughout the region. At less than £10 a head, with wine, we could afford to eat out most days.

We made our way to Urbino across corrugated hills and misty valleys, their ploughed fields the uneven colour of cappucino blanched with cream, their heights overtaken by chestnut and pine woods and beetling towers. Urbino's 15th-century Duke of Montefeltro planned a state based on the Renaissance principles of rationality and humanism. His city-palace, instead of being designed on the old lines of attack and defence, was built to reflect the new humanism, open to the circulation of people and ideas.

An art historian friend of mine, describing the harmonious result, built in creamy travertine with soaring walls, slender pinnacles and a perfectly proportioned courtyard, was moved to tears: "The most perfect fusion of architecture and the human spirit I know."

The rest of the town cascades downhill around the palace's skirts, its suburbs remarkably free of 20th-century development. We visited Raphael's house, a charming domestic interior, full of light and unexpected spaces. Early 15th-century frescos in the empty oratory of San Giovanni depict the Marchegiani carrying on regardless, on the fringes of great events: John the Baptist baptises while a picnic party roisters in the background, a child cuddles a puppy, a man dries his feet before putting on his socks.

"Nothing changes around here," said an old man presiding over the public loo, who requested my help in putting up a poster. "Without tourism, we'd be sunk."

Driving south from Urbino back to Montecarotto, we longed to leave the road and lose ourselves in the beckoning medieval alleys of Mondavio, Corinaldo and Ostra Vetere. Like Fossombrone, Chingoli, Fabriano and Macerata, every one of them deserved a day to itself. But you can't hurry in Marche. "Stai tranquilla" ("Go tranquilly"), said a man in a designer sweater of whom I asked directions in Jesi. There doesn't seem to be any other way to go.

Our house, once a cattle shed and peasant home, had been converted with loving pride by Carlo and Anna, a reflection perhaps of things to come in Marche. Its stunning view, its old furniture and embroidered linen sheets, its day-long scuffle of swallows in the eaves, were as much of a lure as anywhere else.

Jesi, a drowsy, cinnamon-coloured town, pocketed with unexpected patches of garden and perilous stretches of wet cement, has wide squares sporting a wonderful Renaissance town hall, a handsome theatre and a church with a window shaped like a flying hourglass. Time makes odd leaps in Marche. No one looking at the 15th-century frieze of funny-faced moons painted below the Capellone frescoes in Tolentino's splendid Basilica could be surprised that today this is a centre for caricaturists.

The great shrine of Loreto, like most pilgrim churches, is filled with purpose and bustle: busloads of ladies brandishing fat candles like rolling pins and lots of floor-mopping and wheelchair-aligning by Edith Cavell lookalikes. But inside the Holy House, miraculously transported from Nazareth by 13th-century angels, there is utter, clock-stopped silence. The hunched elderly man, the woman on crutches, the mother with her arms round her child lean on the walls for comfort, all eyes fixed on a small black Madonna hung with jewels.

If you espouse the twitcher approach to sightseeing, Marche's laid-backness can have its downside. Much of the information (15lb of it) sent to me by the region's UK representative bore little relation to anything as prosaic as opening times. In Ascoli Piceno, which has one of the most exquisite open-air drawing-rooms of a square in north Italy, hardly any of the 12 churches that the brochure besought us to visit were open. In the picture gallery, we asked directions to the Crivelli exhibition. "None here," said the custodian vaguely, with sublime disregard for a large advertisement downstairs.

Sassoferrato sleeps on its feet. A Franciscan friar jerked a nicotine-stained thumb in the direction of the convent when we asked about two paintings of the Madonna by the artist who took his town's name. A request through a grille set in a solid oak panel eventually produced a tiny, whiskered nun. She fetched the paintings for us, tottering under their weight as she displayed them. "Which do you like best?" Her old finger hovered perilously close to the painted cheek. "This one, with her look of sorrow, is my favourite."

Much of the pleasure of an area so free of tourist traffic lies in simply being with Italians doing Italian things. As we tagged along behind a primary school group visiting the brooding, 1,000-year-old monastery of Fonte Avellana, we admired the Benedictine brother's style. So did the teachers; he had their charges eating out of his hand. "What's the first rule of this monastery? Saying your prayers? Certainly not! It's spitting out your chewing-gum in that bucket. What did Dante Alighieri write when he stayed here? Pinocchio. Oops! Sorry, I meant Paradiso (Canto 21)."

We listened to the town band oompahing in the square of Nocera Umbra in honour of Labour Day and later joined hundreds of families picnicking by the shores of Lago di Fiastra. Innumerable Marche folk festivals are for real folk: Ascoli Piceno's medieval joustings, Offida's July lace fair, the ham and wine fest in Ripatransone in August, the sword race and Palio in Camerino in May.