Without giving them any context, as the class to give you a person, place or a thing (literally anything that's school appropriate, except for students or teachers at your school).
Use a bracket creator generator to make a bracket you can print off - bonus points for making it poster size to hang in your room. This website is easy to use: https://www.freebracketgenerator.com/
Each day have a quick (4-8 measure) playing quiz. Any kid that plays it correct gets to vote in that days match-up. Winning "thing" moves on in the bracket. After a week or two, depending on how big your bracket is (I would do no more than 16 "things", but 8 is probably best).
My all time favorite match-ups have been Spongebob Square Pants vs. Pizza and A Cactus vs. Axe Spray