For a richer flavor, consider using dark brown sugar in the streusel. Toast the walnuts lightly before chopping to enhance their nutty flavor. If you don't have sour cream, plain Greek yogurt can be substituted. Store leftover coffee cake in an airtight container at room temperature for up to 3 days or in the refrigerator for up to a week. Feel free to add other spices like ginger or cloves to the batter for a warmer flavor profile.