One of the things I love about London is that when you set of not entirely knowing what you want to see, you are sure to come across something interesting and new.
My evening weekend walk randomly ended up at Tate Modern and completely surprised me with Art In Action that is on until the end of October. You can lay down on the floor with a cushion under your head to watch colourful visuals. Play with animations on iPads in the rhythm of electro, or just have a pic of different artists and photographers using the space for their projects. Very relaxing and a bit surreal experience. Have a look when you are in the area!