For my last Colonoscopy they gave me 24 tablets of some medication (~~I can't recall what it was~~) Sutab, that I needed to take around 1800 the night before, along with, perhaps, 16 ounces of water total.  Essentially no dietary restrictions beforehand.

That was the best bowel prep I have encountered - super simple, although the tablets were huge!

I think if we used that method more, or something similar to it, instead of telling people to drink 1.5 gallons of something awful, we'd have better compliance.