K21 is one of my favourite art museums. It keeps bringing me back to Düsseldorf every year. Uwe Kiessler repurposed the former Ständehaus, which was reopened in 2002 to exhibit contemporary art including the recent large-scaled installation Die Treppe by Monika Sosnowska:
Tomás Saraceno's Labor project challenges the traditional idea of space, time and gravity. His very original "cloud cities" are floating cloud-like cell towns with a structure of soap bubbles existing as individual cells yet also capable of connecting to each other.