you can draw on your lens with permanent marker, and it'll not show up fully in the video, but it's there. it causes some wicked subliminal messaging!
you could also tape a kaleidoscope to the viewfinder. when you look through the viewfinder kaleidoscope, it's super trippy. unfortunately, when you record, it doesn't record the kaleidoscope, so for true results with the kaleidoscope in the recording, you have to video record through the kaleidascope viewfinder with another camera, ya dig?