Solve problems mostly. They had to build a bridge they could stand on using only paper and popsicle sticks. The goal was for the kid to get as high off the ground as possible without it breaking, and yes, the kids really stood on them!
They built a mini-catapult out of rubber bands and dowel rods that launched ping-pong balls.
They made a booklight that really works, and that involved soldering, even.
They made a solar cooker out of cardboard and aluminum foil.
It was good stuff!
The guy is a legend in our area. He does something different every year, too, so kids can retake the class and still love it.