On the one hand, the Grosser Garten (Large Garden), Berggarten (Mountain Garden) and Georgengarten (Georgian Garden) are prime examples of why Hanover calls itself a city ‘in green’. On the other, the statues, fountains and coloured tile walls of the Niki de Saint Phalle Grotto (opened after her death in 2002) provide a magical showcase of the artist’s work that could one day outshine Die Nanas