The swallows have swooped in, roses have burst into flower—and the Serpentine Pavilion has sprouted in Kensington Gardens. There may be other signs to the contrary, but these three arrivals announce that summer, unequivocally, is here.

The Serpentine Pavilion: a dazzling addition to Kensington Gardens.

Anything involving London’s Serpentine Gallery is always an event. Its exhibitions invariably catch the zeitgeist: thought-provoking, off-piste, fun…I rarely regret the pilgrimage to check them out.

And what better advertisement hoarding could the gallery erect than its pavilions, temporary structures designed by international names which have popped up here every summer since 2000. Following a parade of wacky designs by starchitects like Jean Nouvel, Frank Gehry and Oscar Niemeyer, for 2013 it’s the turn of a comparative unknown, Sou Fujimoto from Japan. Knowing the Serpentine’s knack for backing winners (I recall an exhibition years ago with a pickled sheep by a young artist called Damien Hirst) I think we can be assured that Fujimoto is One To Watch.

I like to use my annual pavilion foray as an excuse for a stroll through Hyde Park’s glorious swathe of green. This year I set off from Lancaster Gate and plunged straight into the freshly restored Italian Gardens. Their spouting fountains and urns with tumbling flowers made an exuberant start to my stroll.

Next landmark was the Henry Moore Arch, now happily back in place after it became unstable and was off show for 16 years. It looks terrific reflecting in the Serpentine, framing the vista towards Kensington Palace. Just beyond is The Magazine, an historic former munitions store, which is being transformed by Zaha Hadid into the new Serpentine Sackler Gallery. It’s due to open later this year with exhibition space and a cafe. Mark your cards for the opening now!

And so to Fujimoto’s pavilion, a non-building or at least one that seems to have vanished, leaving only its shimmering scaffolding behind. You can see it as many different forms: billowing mist, a mass of phone masts…but it struck me as a magical oversized child’s climbing frame. I’ve never seen so many grown-ups clambering around with such glee, beaming down from the highest points as if to say “I’m the king of the castle”. I stepped gingerly from one glass box to another, never sure whether I was planting my foot on thin air, and confess to feeling quite superior when I secured a lofty perch.

Rock on Top of Another Rock outside the Serpentine Gallery.

Rock on Top of Another Rock eventually drew me on my way. This teetering monument by artist duo Fischli/Weiss is so dramatic in this natural setting it’s a shame it will move on next March to the Middle East.

And so to a waterside coffee among yellow irises fringing the Lido as a bunch of swimmers battled the waves. And along the lake to The Dell and Rose Gardens, bursting with colour and scent. At Hyde Park corner my carriage awaited—on the Piccadilly line.