Spongebob - Mermaid Man and Barnacle Boy Bran Flakes

Mermaid Man and Barnacle Boy Bran Flakes are a type of breakfast cereal. It is only seen in the episode, "Mermaid Man and Barnacle Boy II."

It is a light brown box with the modern versions of Mermaid Man and Barnacle Boy on it, suggesting it is either a more recent product or the box art simply changed when the heroes came out of retirement.

There is also a large cereal bowl that resembles Mermaid Man's trunks and belt that is probably a tie-in to this product. SpongeBob also eats this cereal with Breakfast Bites which is likely another tie-in.

At the start of the morning, SpongeBob eats them while he watches the Mermaid Man and Barnacle Boy episode. He mostly likes them because Mermaid Man and Barnacle Boy are on the box. 

 Bran Flakes is similar; they're possibly the same thing but with Mermaid Man and Barnacle Boy on the box, or else made by the same company/manufacturer.