Winn-Dixie has a free coupon for a box of cereal from their store brand, SE Grocers.
For this one you’ll need to have a Winn-Dixie loyalty account (free to make) and then clip the coupon to save it.
Whcn you check out at the register, just enter your phone number and the coupon will be automatically applied.
It looks like it works for any cereal from their brand. When I followed the link the coupon was right at the top, but if you have trouble finding it the category it’s listed under is Grocery.