Hi,
You can't go wrong with single prints that move such things as gears or gyro's.
https://www.youmagine.com/designs/arc-gimbal
We used this one at Makerfaire UK and is a short print and kids, teens and adults loved it because they don't get how it prints the mechanism.
https://www.youmagine.com/designs/modified-space-wrench
NASA space wrench is always a good one for the same reason and it's printed in space.