Free delivery through Denny's on Demand comes in the form of a weekly coupon sent via email through the end of the year (technically through January 3, 2021). Each coupon is valid for 7 days and require a $10 minimum purchase to redeem.
Other deals Denny's is currently offering include $5 off an order of $20 or more when you use the code "5OFF20" (through July 31, 2020) and $5 off a Build Your Own Cheeseburger Family Pack when you use the code "FAMILYPACK" (limited time with no definite end date as of this writing).
Photo via Denny's.