An educational computer you can watch think. It is built from discrete transistors, diodes, resistors and LEDs, with no integrated circuits in the processor. Nearly every transistor has an LED on its collector, so the whole state of the machine is visible: the registers, the bus, the arithmetic unit and the phase of the instruction being executed.
A Raspberry Pi Pico 2 provides the memory and the clock. Because the Pico makes every timing edge, the machine runs correctly at any speed from one phase every hundred seconds up to tens of kilohertz. A class can step through a single instruction one phase at a time and see exactly what moves where, or let a program run.