20 questions is a good warm-up game that doesn't require any planning! Just think of something that you're sure your students already know. I normally think of something easy, like apple or elephant. Tell them they can only ask yes/no questions and have them raise their hands to ask one at a time. I normally answer their question and then write it on the board so they can keep track, for example, "it's a food," "not a fruit," and so on. So far, the middle schoolers I work with have never needed more than 15 questions to nail it.
For younger students, easier questions like "Is it (adjective)?" Are great. For older students, you can use harder objects and encourage harder questions like, "Can we see it (in the school/at the store)?" "Can we use it to (write/travel)?" If they're really good, you can even have a student be one thinking of the item and answering the questions. I think it's very flexible for different levels.
Thank you very much for this!! One of my JTEs has had me do only warm-up games (nothing else, not even human tape-recording lol) for the whole year, so my warm-ups are getting kind of stale at this point. It never even crossed my mind to try doing 20 questions!! I'll try this tomorrow!