We made MICRO SUMO-BOTS with teens at the library a couple years ago, and they loved it. We used toothbrushes (Dollar Tree), small vibrating motors (Amazon, less than a buck each), small, flat batteries (Amazon, packs, .50 or less each), pipe cleaners for balance, and double-sided tape or glue guns. This link shows similar instructions (though they didn’t say use pipe cleaners for balance):

https://www.evilmadscientist.com/2007/bristlebot-a-tiny-directional-vibrobot/

We drew rings on paper to lay on tables when they were done to have sumo matches.

-Post courtesy of Wendy Rowe of the Barrington Public Library