Dig the origins of 10 Food Phrases over at cool site called Mental Floss:

Sowing Your Wild Oats: Avena fatua, a species of grass in the oat genus, has been referred to as “wild oats” by the English for centuries. Though it’s thought to be the precursor of cultivated oats, farmers have long hated it because it is useless as a cereal crop and hard to separate from cultivated oats and remove from fields. Literally sowing wild oats, then, is a useless endeavor, and the phrase is figuratively applied to people engaging idle pastimes. There’s also a sexual connotation in that a young man sowing his wild oats is spreading seed without purpose.The saying is first recorded in English in 1542, by Protestant clergyman Thomas Becon.

A Piece of Cake: The earliest appearance I can find is in Ogden Nash’s Primrose Path in 1936, and the phrase seems to have descended from the earlier “cakewalk.” This second term originates with a 19th-century African American tradition where slaves or freedmen at social gatherings or parties would walk in a procession in pairs around a cake and the most graceful pair would win the cake as a prize (this may also be the origin of “takes the cake”). Although the cakewalk contest demanded some skill and grace, the phrase was eventually adopted as boxing slang and flipped to connote an easily-won fight.