What are you looking for with "amazing" some thing that will wow the crowd, but might not be too impressive to the judges, or something that might wow the judges, but maybe not the crowd? Or better yet, both?
I ask that because there are plenty of projects out there that give that impression of magic, the average person will be blown away, yet the judges likely know the real science behind it... If you go and bore the crowd though you may find it less fun and exciting for yourself.
I suggest to check out this teacher made review that I found recently:
There are all kinds of projects there - good luck!