I was interested in computers at a very early age. They were so science fictional (I loved science fiction) and the idea of one of those big electronic devices (with vacuum tubes and blinking lights, or course) was pretty exciting to a kid.
So, one year, my parents gave me one as a gift: Digi-Comp
Digi-Comp was a mechanical computer. No power source. It was made out of plastic, some metal, and rubber bands. You programmed it by putting a series of one-inch long plastic tubes* on pegs. The location of the tubes determined the programming, and you could do arithmetic operations by moving a lever back and forth. The numbers were in binary, of course, and there were three digits, so you could only get up to seven before overloading the computer.
Here's a nice little Youtube clip. If you look closely, you can see the white numbers on the left side increase each time the computer is cycled.
Digi-Comp was clearly not designed for any serious computing, but it did display the principles nicely. I took it into school to show how computers worked.
I did grow tired of it, mostly because moving the tubes around was always difficult and slow, and debugging involved making sure they were all in the right place.
But it was a great toy while it lasted.
*Just like the Internet!