Teach them to count in binary. I did a career day once where I did this. It might have been more 4th and 5th grade kids, though.
I started by asking the kids how computers kept track of numbers. Of course they had no clue. I said that computers only had electricity to work with, and asked what was the simplest thing that could be done with electricity. By toggling the room lights a few times, I got to on-and-off. Next I talked about using on-and-off electricity as a code. I had four red 25-watt light bulbs fastened to a board and asked with those four bulbs, if each could be on or off, how far could we count. Of course "four" was the most common answer. I told them in fact that we could count 0-15. I reminded them of how we count in base 10; most are familiar with the "ones place", "tens place" etc. Next I dropped cylinders around each bulb on which I had printed large numbers, 8-4-2-1 (these were just 8.5" x 11" sheets rolled into a tube and taped). I introduced binary by saying computers don't have ten fingers, they only have electricity, which can be on or off, so base 2 instead of base 10. Easy to lose them at this point, but talk about just having two digits, zero meaning the light (electricity) is off and one meaning on.
I had a counter circuit driving the bulbs with triacs. (Some high-brightness LEDs would have been easier.) I was surprised how fast they caught on. I wrote the binary equivalent with ones and zeros on the board as I stepped the counter. They all want to push the button to step the counter!
I finished by telling them that they were all now computer experts, because they understood the binary language, and that they should be sure to go home and teach it to their parents that evening
The other thing that they just loved was tearing down computers. I procured a couple old desktops from work that were destined for the scrap heap. I pointed out and described the main areas, CPU, memory, disk storage. Beforehand I had removed the cover from a disk drive as well so they could see the inside. Several of them asked if they could have parts from the computer. So the chassis came back home with me, but were missing memory and other such things as would fit in small pockets.