Spock holds a "marshmelon".
Marshmallows are a Terran confection consisting mostly of sugar and egg whites. They are used in many desserts, including Rocky Road ice cream. (ENT: "Oasis")
Traditionally, camping out includes toasting marshmallows over a campfire. After researching the USS Enterprise-A's database, Spock brought a marshmallow, which he referred to as a "marshmelon", dispenser along for his shore leave in Yosemite National Park with Leonard McCoy and James T. Kirk. After consuming a marshmallow, it is traditional to then engage in a sing-along. (Star Trek V: The Final Frontier)
In J.M. Dillard's novelization (and in the original script William Shatner contributed to) Doctor McCoy played a practical joke on Spock, reprogramming the computer to display "marshmelon" instead of "marshmallow". Although this wasn't filmed, the error in Spock's pronunciation and Doctor McCoy's delight in it are evident in the movie.