Mix-ins. You can choose your favorite mix-ins. Suggestions include (but are not limited to): chocolate chips, white chocolate chips, dried fruit (cranberries, cherries etc.), nuts (pecans, walnuts), cinnamon chips, etc.
Store/FreezeStore] leftover blondies in an airtight container at room temperature for up to 3 days, in the refrigerator for up to one week and in the freezer for up to 1 month. Serve at room temperature or slightly warmed in the microwave to restore the fresh-from-the-oven texture.