My idea consists of the tests that were once done on engines, in order to smooth the problems with pre-ignition (knocking) in engines. The inside of an internal-combustion engine has many interesting and high-speed properties. One idea is engines failures: Parts in the engine, no head-gasket, no oil in cylinder bore, etc. Pretty much anything. I am pretty sure that they did these tests long ago using a piece of quartz acting as a window. I really feel that this could be a really cool profile view of the explosions in a engine (at idle a piston is moving up/down 12 times a second!) High velocity with smoked-air going into a clear intake manifold would be good too! Check out the video at:
http://www.youtube.com/watch?v=DOdKlNDndOo