A cellular simulation, like the Game of Life, only with typed marks that respond to type-specific rules.
I've collected my cocoa worlds before. website 
Uploaded image
Cocoa was influenced by Java's vision of in-browser computing. Apple distributed a play-only version of the interpreter. I assembled a list of my better worlds and distributed them online. webpage
Elevator Simulator moved people between floors at random.
Binary Fibonacci where bits fall into registers and carry when the stack up.
Tube Races were inspired by the reagent limited growth of microtubules.
Portland Pattern Generator based on random assembly of stamp patterns used in the PPR website.
Dannyium Zanide simulated a chemical reaction using element names suggested my my son's friend Danny.
Gimme Five squirted bits that looked like balls into a world that counted them by taking two at a time and converting them to one "carry" bit in the next place.
Nitrogen duplicated the electronic structure of an atom.
Willie Smilies simulates two populations competing based on random motions and collisions that yield happy or sad instances.
Systolic Stack implements spacing rules that insure either a push or a pop can be realized in a single operation without global consequence.
See also HyperCard for more lost work based on Apple products.