An awesome idea would be a fighting robot. Sure, they probably have one, but not one that destroys the other. So they can listen to you if you say "punch" or "kick" and they will do that. Maybe parts could fall off so that they know who wins. Kids would go crazy for that.
Other robots from Synthiam community

Ezang's Following, Tracking The Blue Line With ARC
Thanks for teaching us how to track - used ARC camera movement tracking, sorry for the squeaking, had the tail upside...

Steve's Lynx Robot 1996? Reborn W/Ez Robot
This is my latest project. Vintage Lynx Robot Arm(1996?) reborn with EZB Attending a HAM Fest in Peoria, Illinois,...

Robohappy's Adventurebot4wd
Decided to give the robot a little boost, so im gonna do some reconfiguring. First up, Ive decided to give it sorta a 4...