Just plain water, or broth if I’m sodium deficient. I fast anywhere from 24 hours to 120 hours (5 days) and it works great.
I used to allow breath mints and sugar free drinks, and it did work - but I had more cravings. When I switched to water I stopped daydreaming about junk food, haha.
I’d recommend avoiding skim milk or creamer. If you need something in your coffee, heavy cream works better, since it’s almost pure fat. Fat doesn’t interfere with fasting as much as protein or carb will. Good luck!