(It was Trek on the TRS-80 though that put the hook in me.)

The instructional manual makes a lot more sense with the actual system that it describes in hand.

I imagine someone has made a CARDIAC emulator web page, but the brilliant idea (assuming I am understanding it correctly) is that you are the CPU, or at least the control logic and datapath (you execute the instructions, advance the program counter, update the registers, handle simple I/O, etc.) Perhaps the operation instructions printed on CARDIAC's cardboard are its "microcode".

Human-as-datapath is a fantastic idea for learning the basics of not just programming, but of microarchitecture. Once you start thinking, "hey, I could make a machine to do all of this stuff that I'm doing by hand" then you are on your way.