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A giant crystal bowl floats in the river. At one end, the open bowl inverts into a huge glass dome that bubbles up from the water's surface. Two suspended glazed tubes link this double structure to the river banks on either side.

With this extraordinary sculpture-cum-bridge, Austria's second city of Graz makes a fitting debut as this year's European city of culture. Its creator is the 63-year-old New Yorker Vito Acconci, whose architectural practice, Acconci Studio, is dedicated to putting public participation back into civic spaces. Much more than just a stunning image, the structure performs several functions that coalesce into a suitably inspirational and richly layered cultural experience.

The structure's basic task is to be a pedestrian bridge across the River Mur. In mid-stream, it blossoms into the surreal vision known as Mur Island. The bowl bit is lined with banks of seating and serves as a performance amphitheatre. The dome that merges into it at one end houses a cafe–restaurant. And the ridge that connects bowl and dome is a children's climbing frame, a backdrop to the amphitheatre stage and the ceiling of the cafe.

The structures are composed of space-frames of tubular steel, with acrylic infill panels that are transparent in the amphitheatre and perforated-opaque in the cafe. Daylight streams up through the amphitheatre from the water surface below, and down through the roof of the cafe from the sky. And the multitude of triangular panels offer a kaleidoscope of views of the river, its banks and the cityscape beyond.

"The different functions should not be separated radically," says Acconci. "Around the island flows water, and we wanted to construct an object that is also flowing and changeable."

Mur Island sets an inspired standard for the 100 other projects Graz plans to carry out this year, including the Art House, which will masquerade as a huge air bubble on concrete stilts, designed by professors Peter Cook and Colin Fournier of University College London.