In the first Rayman game, plums became available when obtaining the fist power. Rayman would punch them out of their trees and ride on top of them. They can be used as boats. Also, if a plum falls on a tall safari hunter's head, he becomes a "plumhead"; a man with a plum for a head and can be stood on.
Plums are used to surf across large oceans of lava. When shot by Rayman's fists, they would bounce around.
Plums are carried by Rayman to throw on top of sharp poles in order to create a platform to jump on. This is their only real role in the game, however, as the rest mainly focuses on its juice form. Andre seems to love plumjuice, and when he is swallowed by Globox, Globox is forced to drink it. However, Globox is allergic to plumjuice, and it causes him to get severely drunk, causing him to create bubbles in the air for Rayman to bounce on, and to fly through an entire level.