A robot demo would be my choice. You need to have a machine, a program they help construct, and a simple goal. The program needs to respond to observed state a

A robot demo would be my choice. You need to have a machine, a program they help construct, and a simple goal. The program needs to respond to observed state and make decisions (umm... that means an if statement). There can be all sorts of hidden mechanism, but the exposed mechanisms have to be very simple and clear. You need to actually have a program and a machine to run it on because the magic comes from having the abstract algorithm they help construct run without human intervention. That's the take-away from the whole exercise. You will be surprised how much 2nd graders know. They are from the generation who knows how to login to a laptop and find their favorite videos with no help from parents. And when you get stuck playing a game on your smartphone, the quickest and most correct answer usually comes from asking the local 2nd grader.