Flavorful Bread?

Flour, water, yeast and salt the basic components of the bread I bake. Sometimes I add a little olive oil. My bread is frequently bland. I generally cold ferment my dough for a few days and lately been baking in a enameled cast iron Dutch oven which creates a nice crust. I don't want to add herbs or seeds for this basic bread. Without adding any laborious steps are there any techniques or ingredients I can add to my simple loaf to make it more flavourful? Does the brand of flour make a significant difference? I generally use bread flour, Gold Medal or King Arthur.