Anything, complicated too, but it should have something to do with Redstone
You could build a (semi-) automatic farm
Build a machine that brings your world to crash xD
Try to build a 9x9 redstone door.
If it is supposed to be very complicated, use a working calculator.
It's not that difficult, but with a lot of people watching, piston, sand and funnels you can build something like an earthquake simulator. Just try a little with the circuits