Can we have our ball back please?

For a real arty party in the park, just put Rem Koolhaas in charge of the marquee

A lazy, hazy summer's afternoon in Hyde Park: foreign tourists are showing off their £300 loafers, local loafers are showing off their 20-stone beer-bellies. And a big balloon has appeared in front of the Serpentine, its pale white curve poking over the trees like a modern-day St Paul's.

It's the Serpentine's 2006 pavilion, designed by Rem Koolhaas and Cecil Balmond, with Arup, and the private view opens at 6.30pm (the Serpentine's summer party is on Tuesday). Arriving on the dot of half past, your correspondent is miffed to find the pavilion already rammed full and drunk dry of champagne. The art-erati are quick on the uptake when it comes to freebies: only a paying bar remains. Still, the pavilion is a nice enough environment to drink in: consisting of a translucent circular wall (the basket) and a big ball suspended over it (the balloon), it's airy and light. Though you wonder what it could be used for other than housing a host of beautifully shod, extravagantly eye-weared wine-tipplers. The only seating is the occasional black foam block and there doesn't seem to be a surface on which you could show films, for instance.

I meet Chris, who's from Arup. He's spent the last week getting the balloon up into the air. It's full of helium, apparently, and is held down by pulleys and rope. It floats about a metre and a half above the top of the wall; you can see the blue sky through the gap. 'This is the foul weather set-up,' says Chris. 'The balloon will rise so there's five metres between it and the wall.' The weather remains resolutely unfoul. 'Yes,' says Chris. 'But the mechanism isn't quite working yet.'

Arup is a company that specialises in realising enormous structures. It made Anish Kapoor's big red trumpet in the Tate Modern and was responsible for the giant World Cup football boot currently gracing Berlin. What does Chris think the 2006 Serpentine Pavilion will hold, once the drinkers go home? 'Twenty-four-hour intellectual debate,' he deadpans.

Chris is dressed in a normal manner. There are plenty here who are less so, including my friend Charlie, who arrives in luminous T-shirt, cheeky denim shorts, ankle socks and brogues, carrying a white leather man bag and an umbrella. He does not stand out. Many men here are sporting the Euro-trouser (pastel-hued jeans); I spot one in a shorts-suit. Women's fashion ranges from the expensive (Roland Mouret dresses, Louboutin shoes) to the eccentric (emerald trouser suits, scarlet hair, mismatched animal prints). The party isn't as dressy nor as celeb-studded as the summer party, though; tonight is for gossiping (about the resignation of Robyn Archer, artistic director of Liverpool's European Capital of Culture programme) rather than being gossiped about.

When it's dark, the balloon will be lit from the outside; when it's daylight, the light's nicer within. And there's a rather pretty central frieze, like Seventies wallpaper, suspended above the basket, but beneath the balloon. Still, you can't help feeling it would have been better if the wall was slatted; if you could stand and drink and see the park beyond, with the balloon squatting scarily above your head. Much of the Serpentine's charm is in its garden surroundings; that's why its parties are always so glamorous. As we leave, a woman in full cocktail wear, face Botoxed into a perma-sneer, clatters up the metal stairs in spike heels. If she took one of her shoes off and threw it hard enough at the balloon, the party could collapse into Minnie Mouse-voiced hilarity. Still, she's not the best-dressed. That title belongs to a six-year-old boy, in full Superman costume, complete with blow-up muscles. Someone lend that kid some helium, and we'll believe a child can fly.

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