Hi there,
I work at a education toy store in Australia. I need to build a robot that will get people interested in the vex iq and asking questions. We previously built the claw bot, which I personally found a bit dull. What design would you recommend and why. Please remember we have kids running around the store so preferable nothing that shoot projectiles.
Thank you for your help!
I built a ball sorter for an education show over here in the UK. It was made to be cyclic so the balls get transported around the system and at the end, are sorted into tubes of the same colour. Once the tubes were full, the balls would release again and the cycle goes on. People seemed to be happy to sit and watch it going round and round!
Balls go down a ramp and were released a few at a time to a modified Armbot. Then moved to a hopper where they went up an elevator and into the ball sorter which moved on the linear slides to dispense the ball into the correct tube.
See the Projects link below for details.
https://www.rapidonline.com/Catalogue/CmsPage/2108761
A few models on the VEX IQ SnapCAD Repository page include build instructions.You might try looking around there. 
Sumo robots tend to be simple and interesting…although this would work best with multiple kids driving at the same time.