With my daughter, one surprise was that she was fascinated by one of the simplest exhibits – a large magnet with washers. She would have stayed here for hours if I hadn’t insisted we see other exhibits. I was surprised by this (though in retrospect I should not have been). The exhibit is not as flashy or complex as most of the other exhibit. In fact, nothing moves or lights up. But my daughter was simply fascinated by it. In part, I think this was because it was easy for her to engage with (she could reach it and grasp the washers) and there was no right or wrong way to explore. She could build things!
Personally, I was most impressed with the new Tinkering Studio (http://tinkering.exploratorium.edu/) - a place where children and adults can tinker with everyday and not-so-everyday materials.
More exhibits in the Tinkering Studio. The first is a pin ball machine that can be altered just by moving around the cylinders, bells, and blocks of wood. The other is an assortment of circuit components that can be connected to make buzzers sound, run motors and light lights.