For Beijing's Olympic stadium, he helped conceive a bird's nest. For the Liverpool Biennial, Ai Weiwei has come up with a giant crystal-studded spider at the centre of a web, made from a kilometre's worth of steel cabling and LED lighting suspended 18 metres above the ground. After dusk, the effect is of spun silk that glistens with dew, a web expanding from the centre like the sprawl of city lights seen from a night flight.
Web of Light's scale and audacity may elicit envy in Tate Modern's turbine hall. The advantage of its location at Exchange Flags, behind the town hall, is that the public can walk under it in one of the city's grandest squares. By accident the new and shiny spider is strikingly juxtaposed with the square's dark and melodramatic statue of Nelson with sword, flag, cannon and crowns, wept for by Britannia and clawed at by Death's skeleton. The strands of the web echo chains that bind muscular figures beneath the naval hero. Contrasting with the radical Chinese artist's all-enveloping metaphor, Nelson is inscribed with the literal command: 'England expects every man to do his duty.'
Part of Exchange Flags is still boarded off for construction work, but that seems fitting: this is a city going about its business, and the art will cope just fine. Like its spider, the Biennial weaves its way through streets and public places more intimately than the equivalent contemporary art festivals of Istanbul or Venice. Stand next to a hornbeam tree on a hillock, in view of a busy road and a sign for Toxteth, and find the grass moving beneath your feet on a turntable embedded in the earth by Diller Scofidio + Renfro. Peer through the window of an abandoned, burnt out house in the city centre and see a comfortably furnished middle class apartment, complete with breakfast remnants, half-read newspaper and unexpected sunset, intricately designed by Italy's Manfredi Beninati. Walk next door to discover a derelict site transformed by Tokyo architect Atelier Bow-Wow into an open-air theatre, a platform for people-watching.
There is playfulness, ingenuity and joy in these works, rediscovering the wit of a city whose public face is often associated with monumental civic buildings or hoodie-haunted deprivation. Wisely, the Biennial, Britain's largest festival of contemporary visual art, has not sought work that plays up to Liverpool (no laboured Beatles or Blackstuff) or to its current status as European Capital of Culture (there are 40 artists from all over the world presenting almost entirely new commissions). Instead this year's theme is 'Made Up', billed as an exploration of the power of the artistic imagination, surely one of those rubrics so broad as to be virtually meaningless. Fortunately, the artists appear to have taken it in myriad directions. The consequences, inevitably, range from the banal to the beautiful, from the derivative to the daring.
At Tate Liverpool, Canadian David Altmejd has turned a gallery into a giants' lair of fairy tales that could have been borrowed from the dark imagination of Guillermo del Toro. In The Holes, two sleeping giants lie on a table, never to awaken: their decaying bodies are stage sets for images of sex and death. One giant is severed at the waist, its intestines spiralling into an abyss. This weird, operatic landscape also has snails, quartz crystals and mirrors, pastiche pine trees and sunflowers. Walk around it, peer up close or look from afar, and you're still slightly baffled. The final effect is less than the sum of its parts.
Drawing, photography and video are all represented, but few works are more potent than Romanian Adrian Ghenie's paintings, concerned with the peculiar aesthetic of cinema. Nickel Odeon takes the film comedy motif of the pie fight, as seen in Blake Edwards's The Great Race, and merges it with documentary photographs of elderly Jews being forced to clean pavements with toothbrushes. The tension between real and unreal is visceral. At the opposite wall, visitors are likely to do a double take worthy of Laurel and Hardy on realising they are looking at a diptych called Laurel and Hardy, painted not from the comedy duo's classic films but one of their last appearances when both were old, frail and ravaged by illness. Merged with a Velázquez court buffoon, this is first rate portraiture, stripping away Hollywood's manufactured images to expose warts and all.
Over at the Bluecoat, Australia's Tracey Moffatt gives nostalgia a candy-coloured glow. A series of mundane odd jobs from her youth - corner store, canteen, hairdresser, reception - are recreated in technicolour photos that make the past seem like a gaudy wonderland. With some clever Photoshopping, Moffatt has inserted herself into her own sugar-coated memories. On a different note, her video Doomed skips between clips from disaster movies accompanied by a pounding electronic soundtrack. The result is fascinating, disquieting and likely to make the heart race.
Like most, but not all, of the artists on show, Moffatt is mindful of her duty to grab you by the lapels and entertain. Tate Liverpool's Gustav Klimt exhibition, the Walker Art Gallery's Art in the Age of Steam and La Machine's 50ft mechanical spider that paraded in public earlier this month are making it a record-breaking year for art in Liverpool. The Biennial, now a decade old, is set to make it a vintage one. Just remember to get a map.