RodRego (visiting click in ‘i’ for a hint) – a simple toy, so called Register Machine, but the real educational help. I’m much impressed that it was developed by a philosopher for his students. The simplicity remainds me Core Wars of ’80. By the way, how many present coders have ever tried an assembler, machine code? 
More food for your thought to find in a recommended “Intuition Pumps and Other tools for Thinking”* by Daniel C. Dennett

*) “Dźwignie wyobraźni i inne narzędzia do myślenia”