Part of the problem in answering this question is its vagueness. Is your client a film producer? An ad agency creative director? A theatre owner? A club owner looking for a DJ? A museum curator looking for the next interactive exhibit?
Your demo reel depends on how you answer the above. For film or TV, I'd recommend finding an exciting sequence from a movie or TV show and creating new sound design that is compelling, original and unique. Try to stay away from clichéd ideas and stock library sounds. The people judging your work want to be pleasantly surprised, and they don't necessarily want to watch the entire 2:30 demo to the end to get that response.
Designing to particle fountains or colors? Eh, that really depends on your target audience. For theatre or museums, maybe, but I believe most potential clients/employers want to experience something they can relate to.