I love miniball bots and other types of ballbots where the electronics and mechanics are housed inside and the vehicle “navigates” by rolling around and bouncing off of stuff. Here’s the same basic idea, inside a cylinder (a clear paint can). It’s the Barreller, by Randy Sarafan.
The way the continuous rotation servo is used and the use of the paintbrush handles as the “eccentric” weight are innovative. This is just one step up from a basic vibrobot, in terms of build complexity. And like a vibrobot, this would be a fun project to do with kids. It’s just complicated enough for them to feel like they’ve really accomplished something, while being quick enough to maintain their attention.