After going through some simple flash cards such as colours, animals, food or body parts, give each student a small card with one of the words on it. Tell them you are going to say a word and whoever is holding that word is "it" (oni in Japanese). They have to tag someone before you shout the next vocab word. If a student gets caught, they swap vocab cards and become the "oni". There is no winner or loser in this game, so there shouldn't be any tears.
See more information about how to get started at the Central Wikia tutorial.