I like just adding in frozen fruits and vegetables. Try Mixing flavors that you love. Also adding thing that don't Impact the flavor that much is a great way to sneak in health nutrients. I always love adding spinach.
A diced up banana, 1 to 3 tablespoons of peanut butter, Some oats, A pinch of walnuts, Handful of spinach and a dash of honey. A scoop of protein powder is optional. Top it up with some milk, then mix well and enjoy.