A common thing in a lot of enema fiction is a soapsuds enema. I have to say that you should NEVER use ordinary soap for this (hand soap, shampoo, dishsoap) as they irritate the bowel and often contain anti-microbial agents that can damage the bacteria in your intestines. Also, most contain sodium laureth sulfate which is thought to be carcinogenic, although not proven.
If you have to have a soapsud enema, use a plant, animal, or other food-based soap. Some examples are the very popular Castile soap and goat milk soaps.
A popular additive that was bragged to be healthful was the coffee enema, which was know as the Gerson therapy. This is believed to have been the cause of three deaths in the United States.
Okay, fun fact time! The enjoyment of enemas is called klismaphilia. *skips around* I'm a klismaphiliac, I'm a klismaphiliac!