Every summer since 2000 the Serpentine Gallery in Hyde Park has commissioned a temporary pavilion from a world-class architect. This is the 2009 project, designed by Kazuyo Sejima and Ryue Nishizawa, and it is a real delight.
I've seen seven of the nine pavilions and for my money this is the best; some have been quite lumpen and even inappropriate for the parkland setting. I also really enjoyed the 2002 pavilion by Toyo Ito and Arup.
Interesting that my two favorites have both come from Japanese architects.