I never know what to put in my coffee on fast days. I am doing an oldie but a goodie, hot coco mix with coffee.
non-dairy powdered creamer. As for sugar, I don't think that's prohibited, but you can always use Truvia or one of those alternatives which taste almost the same. Fortunately for me, I grew up on non-dairy powdered creamer since before I even became Orthodox because that was all my parents bought due to my dad's health or something.
I've never been one for straight milk in my coffee. It makes the coffee too cold.