For 16:8 fasting it's just water (I like using a sodastream to make seltzer water) and black coffee. Maybe a tea if I want to mix something up. Never any sugar, cream or other things added to coffee/tea. For the 72 hour fasts it's the same but I have a couple cups of bone broth during the day. It's a minimal amount of calories (~100-200 cals) but more importantly helps get more electrolytes, which I've noticed can be trickier to manage while fasting.