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

Kullthulu's Artemis - The Household Service Bot
Hello Everyone! I posted a project a while back, (2014 so a wayyyyy while back) about a robot I was building that at the...

Andy's Happy Holidays
A Holiday Wish from Antonn.

Cliffordkoperski's HAL THE ROBOT THAT WALKS AND TALKS
5.5 foot tall humanoid robot that walks and talks. 56 servos controlled by 3 ezbv4 boards working together over wifi. On...