If you have a toddler and can get to Clapham in the next few days, we would highly recommend Colourscape. It's a temporary exhibit which travels around the country hosting mini-music festivals. But that is not what we were there for!
As part of the Colourscape exhibit, they have a fantastic structure that can be best-described as a series of massive zorb balls all joined together. If you don't know what a Zorb is, it is a big inflatable spherical ball that can fit a person inside. The Colourscape balls are huge though - about 20 feet across inside and each one is illuminated in a different colour. This leads to a magical experience for kids because if you are wearing red clothes and go into the red room, you become virtually invisible! All of the balls interconnect together, so as you keep walking things change continually.
With everyone wearing a coloured cape that is handed out on the door, it is actually quite fun to wander around inside and see how things change. Taking photos with an IPhone is basically impossible - the camera just can't cope with the colour overload inside.
The inside is like a bouncy castle, so it is also a great sensory environment for children to explore. Our toddler loved it.
It is only open for two more days for one hour at lunchtimes as there are private events on at other times. Only limited numbers of people can get in but as long as you are there by midday you should hopefully get in. Ticket price was £4 for adults.