When I had worked with my yearlings in the past, I would always get them used to a phone ring sound. I've had horses spook and nearly run off when my cell phone would ring while riding, so I make sure to get them used to that.
What about walking over things? Such as logs, etc? I think that would be a good one to work on.
It's always nice to work with the youngsters really well and then later being able to saddle break them really easy without much trouble. :)