Kelp rings are a food sold at the Krusty Krab. They only appear in the episode "Mimic Madness," but have been seen on the Galley Grub in many other episodes.
They cost $1.50 and look just like onion rings, with the exception of being made of kelp. They are most likely a side order with the Krabby Patty.
Role in episode
A kelp ring is seen on the ground near a trash can in front of the Krusty Krab when Patrick is shown eating from said trash can.