The very first such premium was released in 1894 when Aunt Jemima pancake mix boxes first came printed with paper dolls of Aunt Jemima, Uncle Mose, and their kids. The oldest examples are cereal and/or breakfast food premiums.
More broadly, a premium is anything you get in return for buying something else.
They don’t HAVE to be toys either: story books, school supplies, sunglasses, cups, and other items have been packed with kids meals over the years. Debuting in 1979, they’re historically a tie-in to a kid friendly movie or TV series that’s the new hotness. The most famous, naturally, is the McDonald’s Happy Meal Toy. In common parlance, it’s the toy that comes with a kid’s meal.
A Pikachu Gold-Plated Card, still sealed in its bag.
