I typically eat in a very low carb/keto way, which IMO is the best way to have a comfortable extended fast. You’ll get into ketosis one way or another, and its very hard to go from your body craving constant sugar spikes to complete fasting without feeling like complete trash. As a result, at least 3 days on keto is what i’d recommend.
Electrolytes, i take them in the late afternoon into the evening. I do try and do 15-20 minutes of moderate-high intensity exercise. Afterwards I drink water with magnesium-citrate and lo-salt. No other vitamin/micro supplements, but I have never done a fast longer than 5 days, and i typically fast 44 hours twice a week.
Coffee, I drink it in the mornings. I enjoy it. I’m not doing fasting for mental clarity, i’m clear enough already ;) but i’ve not encountered issues with my coffee intake.
Foods, sometimes my ADF style fasting i’ll supplement with broth. Making it more of a protein sparing modified fast in that way. It isn’t taxing on the digestive tract. If I’m doing it for autophagy (like the 5 day fasts) i’m not going to consume broth.
I just did a 7 day fast. A couple days prior to starting, I eliminated sugars and starchy processed carbs. I also upped my protein & healthy fats. I keep to water, unflavored sparkling water and black coffee. I also take the extended day fasting electrolytes to keep headaches, fatigue and dizziness away. Also comes in a sleep version on Amazon. Good luck! https://www.amazon.com/Extended-Electrolytes-Himalayan-B-Complex-Vitamins/dp/B08ZYWC5Q8/ref=sr_1_6?crid=1X0N702G7LNI2&keywords=reset+electrolytes&qid=1672843231&sprefix=Reset%2Caps%2C142&sr=8-6
Im doing a 7 day Water Fast and I’d say one prep you can do is to get yourself into Ketosis before starting the fast so those first two days are really productive with fat loss and autophagy. I noticed the first few days (I ate Pizza on NYE and Im not sorry about it) It took a while to get back into Ketosis which I normally am in and the weight has dropped quicker these last 3 days of the fast.