Water
I’m interested in simulating water. I imagine various art installations which try to give the impression of being in water by using light or movement that is responsive to the presence of people within the installation. I like the idea of using a set of lasers which are sequenced under computer control to produce a wave-like surface above the viewers head which undulates as people move about (although this could only produce planar waves.) Another idea involves a ring of blue LEDs which demark the edges of a simulated water surface and are sequenced vertically to give the impression of wave motion.
There is a darkened hallway at the Seattle Aquarium that uses lights to simulate water. Have you seen that? My little 3 year old is afraid to walk through and always runs instead.