Oatmeal. You can customize it to what you need and your tastes. It just needs liquid for the oats and whatever additions you want to do. I use chia seeds and flax seeds for protein & fiber, and add frozen fruit and vanilla soy milk. Microwave and enjoy.
Peanut butter, dried fruit, honey, fresh fruit, etc. are all good choices
I have little meal prepped oatmeal jars I set up. Everything is pre-measured just add a cup of water and pour in the jar, boil and you're done. They have oats, brewers yeast, salt, pepper, garlic powder, and sometimes whey protein (that one changes how you have to hear it though to prevent weird clumping). Throw a fried egg on top of I have time, or a scoop of cream cheese to mix things up