If you have a spare 3 HOURS (!!) and an iPad full of unwatched films/a chatty friend, you should undoubtedly consider queuing for the Rain Room installation. Using seemingly invisible motion detectors, your movement is sensed as you walk through the rain, switching off the outlets above your head so that, despite being surrounded by falling water the entire time, you never in fact get wet.
The wait might seem unbearably long, but this is simply because people find it difficult to leave; and certainly once you have stood/sat for so much time, a whole ten minutes of standing amidst the intelligent showers seems deserved no matter how much the rest of the queue will despise you. I love rain in its usual context, let alone when I can be encircled by it whilst my Junya coat stays dry. The whole experience is incredibly comforting, particularly in the darkness, and although less than 12 or so people are admitted at one time, you only wish to be isolated once inside.
–> @ the Barbican